[M] Change in coreboot[master]: mb/google/myst: Add new mainboard

[Fred Reitberger (Code Review)]

Attention is currently required from: Raul Rangel, Martin L Roth, Jon Murphy, Eric Lai, Martin Roth, Tim Van Patten, Karthik Ramasubramanian, Felix Held, Mark Hasemeyer.

Fred Reitberger has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/74093 )

Change subject: mb/google/myst: Add new mainboard
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Patch Set 3:

(3 comments)

File src/mainboard/google/myst/Kconfig:

https://review.coreboot.org/c/coreboot/+/74093/comment/16ab8ea9_4a17e88b 
PS3, Line 8: config AMD_FWM_POSITION_INDEX
           : 	int
           : 	default 3
Phoenix has a fixed EFS location (0x20000) so this config value has no effect

File src/mainboard/google/myst/chromeos.fmd:

https://review.coreboot.org/c/coreboot/+/74093/comment/028b6d4d_ee665e14 
PS2, Line 2: 	SI_BIOS {
> It seems a mix and match of Birman and Skyrim. […]

RECOVERY_MRC_CACHE should be 256K, same as the size of RW_MRC_CACHE

The EFS must exist at the 128K offset, so the RW_MRC_CACHE will not fit at the beginning of flash

File src/mainboard/google/myst/chromeos.fmd:

https://review.coreboot.org/c/coreboot/+/74093/comment/9b45bcf9_4bebdbd7 
PS3, Line 36: }
> Fred/Martin, […]

RO should cover the lowest flash offsets.  See the birman `chromeos_phoenix.fmd` layout for an example.

As far as 32M layout, we have been testing putting the RO PSP binaries in the upper 16M and the rest of the image in the lower 16M half, but I don't think that will work long-term unless the flash chip supports two different write-protect ranges?