phy: mediatek: mipi-dsi: Convert to register clk_hw

Instead of registering a struct clk, directly register clk_hw: this
allows us to cleanup a pointer to struct clk from struct mtk_mipi_tx.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230525115258.90091-2-angelogioacchino.delregno@collabora.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>