soc/mediatek/mt8192: initialize DRAM using vendor reference code

Mediatek has released the reference implementation for DRAM
initialization in vendorcode/mediatek/mt8192/dramc (CB:50294)
so we want to use it to replace the derived calibration code
in soc folder.

Signed-off-by: Xi Chen <xixi.chen@mediatek.com>
Change-Id: I2b2f41d774c6b85f106867144fb0b29a4a1bdfcf
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51125
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>

src/vendorcode/Makefile.inc

@@ -4,3 +4,4 @@ subdirs-y += intel
 subdirs-y += siemens
 subdirs-y += cavium
 subdirs-y += eltan
+subdirs-y += mediatek