mediatek/mt8183: Add a stub implementation of the MT8183 SOC

mediatek/mt8183: Add a stub implementation of the MT8183 SOC

Most things still need to be filled in, but this will allow us to build
boards which use this SOC.

BUG=b:80501386
BRANCH=none
TEST=timer and uart work fine

Change-Id: Ie81fa56ffce85188e1f9e979f9b0e64b764c2627
Signed-off-by: Tristan Shieh <tristan.shieh@mediatek.com>
Reviewed-on: https://review.coreboot.org/26659
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>