[L] Change in coreboot[master]: device/dram: add DDR4 MRS commands

[Sergii Dmytruk (Code Review)]

Attention is currently required from: Andrey Petrov, Angel Pons, David Hendricks, Krystian Hebel, Martin L Roth, Matt DeVillier, Michał Żygowski, Paul Menzel.

Sergii Dmytruk has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/67059?usp=email )

Change subject: device/dram: add DDR4 MRS commands
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Patch Set 11:

(1 comment)

File src/include/device/dram/ddr4.h:

https://review.coreboot.org/c/coreboot/+/67059/comment/6b36ff34_50860108 :
PS11, Line 106: typedef u32 mrs_cmd_t;
> Nit: We prefer not to use typedefs unless there's a specific reason to. […]

Changed to `uint32_t` and added comments to note that MRS command is being returned.