dt-bindings: i2c: Add MediaTek MT7622 i2c binding

Add MT7622 i2c binding to binding file. Compare to MT8173 i2c
controller, MT7622 limits message numbers to 255, and does not
support 4GB DMA mode.

Signed-off-by: Jun Gao <jun.gao@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>