phy: mediatek: mipi: mt8173: use common helper to access registers

Use MediaTek phy's common helper to access registers, then we can remove
mipi-dsi's I/O helpers.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220920090038.15133-16-chunfeng.yun@mediatek.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>