Change in coreboot[master]: soc/intel/graphics: Create Kconfig for mapping graphic memory base

[Wonkyu Kim (Code Review)]

Attention is currently required from: Nico Huber, Ethan Tsao, Subrata Banik, Ravishankar Sarawadi, Tim Wawrzynczak, Paul Menzel, Raj Astekar, Patrick Rudolph.
Wonkyu Kim has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/61389 )

Change subject: soc/intel/graphics: Create Kconfig for mapping graphic memory base
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Patch Set 17:

(1 comment)

File src/soc/intel/common/block/graphics/graphics.c:

https://review.coreboot.org/c/coreboot/+/61389/comment/53442924_1961e04e 
PS13, Line 128: memory_base = memory_base + CONFIG_SOC_INTEL_GFX_MEMBASE_OFFSET;
> you have marked done but appear not done yet. […]

GTT size is considered in CONFIG_SOC_INTEL_GFX_MEMBASE_OFFSET.  So, just constant offset should be fine for now as we're working on fixed GTT size; FSP will be updated for this.

If we need to support flexible size in total offset in future, we may need to update calculation later.