ASoC: mediatek: add MT7622 AFE support

This patch adds support for the MT7622 AFE which reuses MT2701 driver.

We also introduce the 'struct mt2701_soc_variants' to differentiate
between the SoC generations as there might be other (existing or future)
chips that use the same binding and driver, then being a little more
abstract could help in the long run.

Cc: Jia Zeng <jia.zeng@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Garlic Tseng <garlic.tseng@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>