ASoC: add mt6351 codec driver

This patch adds the MediaTek MT6351 codec driver.
MT6351 communicate with SoC through MediaTek PMIC wrapper.
MT6351 use MediaTek proprietary audio interface.

Signed-off-by: KaiChieh Chuang <kaichieh.chuang@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>