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

[Angel Pons (Code Review)]

Attention is currently required from: V Sowmya, Martin Roth, Tim Wawrzynczak, Balaji Manigandan, Aamir Bohra.
Angel Pons 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 4: Code-Review+1

(2 comments)

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

https://review.coreboot.org/c/coreboot/+/49731/comment/d3227bc0_f9faaa7d 
PS4, Line 8: 	/* DQ CPU<>DRAM map */
nit: this comment refers to `.lpx_dq_map`, should be moved downwards

https://review.coreboot.org/c/coreboot/+/49731/comment/be3d98ed_4a395f7e 
PS4, Line 74: 	gpio_t spd_gpios[] = {
could this array be const?