Change in coreboot[master]: mb/intel/shadowmountain: Add the romstage code

[Aamir Bohra (Code Review)]

Attention is currently required from: V Sowmya, Martin Roth, Tim Wawrzynczak, Angel Pons, Balaji Manigandan.
Aamir Bohra has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/49731 )

Change subject: mb/intel/shadowmountain: Add the romstage code
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Patch Set 3:

(2 comments)

File src/mainboard/intel/shadowmountain/variants/baseboard/memory.c:

https://review.coreboot.org/c/coreboot/+/49731/comment/b2666e15_0277b6a3 
PS3, Line 58: false
enable

https://review.coreboot.org/c/coreboot/+/49731/comment/7d934ae1_f7048f9f 
PS3, Line 78: 	GPP_D15,
unused, needs to be dropped.