phy: mediatek: Move mtk_hdmi_phy driver into drivers/phy/mediatek folder

mtk_hdmi_phy is currently placed inside mediatek drm driver, but it's
more suitable to place a phy driver into phy driver folder, so move
mtk_hdmi_phy driver into phy driver folder.

Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Tested-by: Frank Wunderlich <frank-w@public-files.de>