mb/roda/rk9: Document flash header

Change-Id: I5bd131635340ffa0c6b8979fc8e263fc5f09fdc5
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32025
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/mainboard/index.md

@@ -81,6 +81,10 @@ The boards in this section are not real mainboards, but emulators.
 
 - [MS-7707](msi/ms7707/ms7707.md)
 
+## Roda
+
+- [RK9 Flash Header](roda/rk9/flash_header.md)
+
 ## SiFive
 
 - [SiFive HiFive Unleashed](sifive/hifive-unleashed.md)

Documentation/mainboard/roda/rk9/flash_header.md

@@ -0,0 +1,23 @@
+Roda RK9 Flash Header
+=====================
+
+There is a 5x2 pin, 1.27mm pitch header *J1* south of the BIOS flash. It
+follows the pinout of the Dediprog adaptor board:
+
+    +------+
+    | 1  2 |    1: HOLD 2   2: CS 2
+    | 3  4 |    3: CS 1     4: VCC
+    | 5  6 |    5: MISO     6: HOLD 1
+    | 7  8 |    7:          8: CLK
+    | 9 10 |    9: GND     10: MOSI
+    +------+
+
+Pins 3 to 10 directly map to the regular SPI flash pinout.
+
+There is also a *JP17* around. Ideally, it should be closed during
+programming (isolates the SPI bus from the southbridge):
+
+    +---+
+    | 1 |   1: SF100-I/O3
+    | 2 |   2: GND
+    +---+