dt-bindings: i2c: i2c-mt65xx: Add MediaTek MT8196/6991 compatibles

Add support for the MediaTek MT8196 Chromebook SoC and for its
close relative, the MediaTek Dimensity 9400 MT6991 SoC.

Those chips' multiple I2C controller instances are compatible with
the ones found in the MT8188 SoC.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
[wsa: rebased]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>