Documentation/mainboard/amd: Add padmelon documentation and images

Create documentation on padmelon, including how to program the SPI. Also
include an index.md pointing to the documentation, as currently there's no
maiboard documentation folder for AMD.

BUG=none.
TEST=none.

Change-Id: I1a684c1acd3fb9441df71e2bc0fffa6131148b98
Signed-off-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34493
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Documentation/mainboard/amd/index.md

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+# AMD platforms documentation
+
+This section contains documentation about specific AMD mainboards.
+
+## Mainboard
+
+- [padmelon](padmelon/padmelon.md)

Documentation/mainboard/amd/padmelon/padmelon.md

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+# Padmelon board
+
+## Specs (with Merlin Falcon SOC)
+
+* Two 260-pin DDR4 SO-DIMM slots, 1.2V DDR4-1333/1600/1866/2133 SO-DIMMs
+  Supports 4GB, 8GB and 16GB DDR4 unbuffered ECC (Merlin Falcon)SO-DIMMs
+* Can use Prairie Falcon, Brown Falcon, Merlin Falcon, though coreboot
+  code is specific for Merlin Falcon SOC. Some specs will change if not
+  using Merlin Falcon.
+* One half mini PCI-Express slot on back side of mainboard
+* One PCI Express® 3.0 x8 slot
+* Two SATA3 ports with 6Gb/s data transfer rate
+* Two USB 2.0 ports at rear panel
+* Two USB 3.0 ports at rear panel
+* Dual Gigabit Ethernet from Realtek RTL8111F Gigabit controller
+* 6-channel High-Definition audio from Realtek ALC662 codec
+* One soldered down SPI flash with dediprog header
+
+## Mainboard
+
+![mainboard][padmelon]
+
+Three items are marked in this picture
+1. dediprog header
+2. memory dimms, address 0xA0 and 0xA4
+3. SATA cables connected to motherboard
+
+## Back panel
+
+![back panel][padmelon_io]
+
+* The lower serial port is UART A (debug serial)
+
+## Flashing coreboot
+
+```eval_rst
++---------------------+--------------------+
+| Type                | Value              |
++=====================+====================+
+| Socketed flash      | no                 |
++---------------------+--------------------+
+| Model               | Macronix MX256435E |
++---------------------+--------------------+
+| Size                | 8 MiB              |
++---------------------+--------------------+
+| Flash programing    | dediprog header    |
++---------------------+--------------------+
+| Package             | SOIC-8             |
++---------------------+--------------------+
+| Write protection    | No                 |
++---------------------+--------------------+
+```
+
+## Technology
+
+```eval_rst
++---------------+------------------------------+
+| Fan control   | Using fintek F81803A         |
++---------------+------------------------------+
+| CPU           | Merlin Falcon (see reference)|
++---------------+------------------------------+
+```
+
+## Description of pictures within this document
+
+```eval_rst
++----------------------------+----------------------------------------+
+|padmelon.jpg                | Motherboard with components identified |
++----------------------------+----------------------------------------+
+|padmelon_io.jpg             | Back panel picture                     |
++----------------------------+----------------------------------------+
+```
+
+## Reference
+
+[Merlin Falcon BKDG][merlinfalcon]
+
+[merlinfalcon]: ../../soc/amd/family15h.md
+[padmelon]: padmelon.jpg
+[padmelon_io]: padmelon_io.jpg