clk: imx: add imx8m_clk_hw_composite_bus

Introduce imx8m_clk_hw_composite_bus api for bus clk root slice usage.
Because the mux switch sequence issue, we could not reuse Peripheral
Clock Slice code, need use composite specific mux operation.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>