[XS] Change in coreboot[master]: mb/google/rex/rex0: Enable Fast Vmode for rex0

[Subrata Banik (Code Review)]

Attention is currently required from: Jay Patel, Kapil Porwal, Tarun Tuli.

Subrata Banik has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/75567?usp=email )

Change subject: mb/google/rex/rex0: Enable Fast Vmode for rex0
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Patch Set 1:

(3 comments)

Patchset:

PS1: 
as per fsp upd description, if icclimit is set to 0 then reactive protection feature remains disable.

```
A value of 0 corresponds to feature disabled (no reactive protection). This value represents the current threshold where the VR would initiate reactive protection if Fast Vmode is enabled.
```

we need to perform icclimit override as well as below

VccCORE	72
VccSA	35

File src/mainboard/google/rex/variants/rex0/overridetree.cb:

https://review.coreboot.org/c/coreboot/+/75567/comment/08792ee5_c61a3a48 :
PS1, Line 58: cep_enable
how is this different from enable_fast_vmode ?

https://review.coreboot.org/c/coreboot/+/75567/comment/40d034e9_ed2cc128 :
PS1, Line 60: 	register "enable_fast_vmode[VR_DOMAIN_GT]" = "1"
            : 	register "cep_enable[VR_DOMAIN_GT]" = "1"
as per 640982 doc, gt doesn't support fast mode vr