Change in coreboot[master]: Add mediatek mt8192 dram vendor code

[Xi Chen (Code Review)]

Attention is currently required from: Hung-Te Lin, Nico Huber, Martin Roth, Paul Menzel, Yu-Ping Wu.
Xi Chen has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/50294 )

Change subject: Add mediatek mt8192 dram vendor code
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Patch Set 5:

(1 comment)

Commit Message:

https://review.coreboot.org/c/coreboot/+/50294/comment/c49079f0_eebf29e0 
PS5, Line 11:
> That the code is developed/maintained somewhere else is a perfect argument […]

@paul: sorry for confusion, the goal is to "use reference memory initialization code from MTK official firmware", the coding style is different from coreboot, which may lead to bug fix difficulty when applying official patches.

Also, will split to two patches.