mb/prodrive/atlas: Add Kconfig option to enable SaGv

It turns out that one can use Kconfig options to specify values for
devicetree options, as long as the resulting expression is a compile
time constant. Use this to configure SaGv for Atlas: enable it by
default, but allow SaGv to be disabled manually for convenience when
testing. Enabling SaGv makes MRC train the RAM multiple times, which
takes a significant amount of time.

For further info on SAGV on ADL, please refer to Intel Doc 655258
(Alder Lake Datasheet) section 5.1.3.2.

Signed-off-by: Lean Sheng Tan <sheng.tan@9elements.com>
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Change-Id: I3c6ac25d414122c408f2348d12dba8dce909e567
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67412
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/prodrive/atlas/Kconfig

@@ -16,6 +16,10 @@ config BOARD_PRODRIVE_ATLAS_BASEBOARD
 
 if BOARD_PRODRIVE_ATLAS_BASEBOARD
 
+config ATLAS_ENABLE_SAGV
+	bool "Enable SaGv"
+	default y
+
 config MAINBOARD_FAMILY
 	string
 	default "PRODRIVE_ATLAS_SERIES"

src/mainboard/prodrive/atlas/devicetree.cb

@@ -14,6 +14,9 @@ chip soc/intel/alderlake
 	# EC memory map range is 0x900-0x9ff
 	register "gen3_dec" = "0x00fc0901"
 
+	# SaGv Configuration
+	register "sagv" = "CONFIG(ATLAS_ENABLE_SAGV) ? SaGv_Enabled : SaGv_Disabled"
+
 	# Display configuration (4 DPs)
 	register "ddi_ports_config" = "{
 		[DDI_PORT_A] = DDI_ENABLE_HPD | DDI_ENABLE_DDC,