Change in coreboot[master]: intel/common/../systemagent: Add sa_get_mchbar_base api

[Subrata Banik (Code Review)]

Attention is currently required from: Wonkyu Kim, Tim Wawrzynczak, Paul Menzel, Angel Pons, Arthur Heymans, Michael Niewöhner.
Subrata Banik has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/63615 )

Change subject: intel/common/../systemagent: Add sa_get_mchbar_base api
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Patch Set 16:

(1 comment)

Patchset:

PS16:
> @Subrata, I'm not worried about #2 because if one forgets to enable MCHBAR early the resulting issues will be easy to notice.
>
> I'm pretty sure that the MMIO accesses to "MCHBAR" (when MCHBAR MMIO window is not enabled) would hit DRAM, and IIRC the SA prevents accesses to DRAM before it has been initialized (on Haswell, the system locks up).

Sounds good to me. we can then use CB:63890 as reference for now