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

mtk_mipi_dsi_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_mipi_dsi_phy driver into phy driver folder.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Vinod Koul <vkoul@kernel.org>