[S] Change in coreboot[master]: mb/google/rex: Add CPU power limit values

[Eric Lai (Code Review)]

Attention is currently required from: Tarun Tuli, Kapil Porwal, Sumeet R Pawnikar.

Eric Lai has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/74379 )

Change subject: mb/google/rex: Add CPU power limit values
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Patch Set 2:

(1 comment)

File src/mainboard/google/rex/variants/rex0/ramstage.c:

https://review.coreboot.org/c/coreboot/+/74379/comment/68bbabc2_0d3fffc1 
PS1, Line 17: variant_update_power_limits
> I feel we need this here for variants to set during ramstage.

I still think put a __weak function in the systemagent.c before set power_limits, like variant_update_power_limits(), and change the common name to soc_update_power_limits.