mmc: mtk-sd: add pad control settings for MediaTek MT7620/MT76x8 SoCs

The driver is missing pad control settings (pad delay and pad conf) for
the mt7620 and mt76x8. Although mt76x8 still works well, mt7620 will
encounter CRC error on data transfers.

This patch adds default pad control settings for mt7620_compat.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>