mmc: dt-bindings: Add reg/source_cg/latch-ck for Mediatek MMC bindings

Change the comptiable for support of multi-platform
Make compatible explicit, as MMC host of mt8173 has difference with
mt8135(mt8173 supports hs400 and hs400_tune),so that need separate
mt8173/mt8135 compatible name.
Add description for reg
Add description for source_cg
Add description for mediatek,latch-ck
Note that source_cg and mediatek,latch-ck are optional for some projects,
eg, MT2701 do not have source_cg, and MT2712 do not need
mediatek,latch-ck

Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>