i2c: mediatek: add support for MT7981 SoC

Add support for the I2C units found in the MediaTek MT7981 and MT7988
SoCs. Just like other recent MediaTek I2C units that also uses v3
register offsets (which differ from v2 only by OFFSET_SLAVE_ADDR being
0x94 instead of 0x4).

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>