clk: imx: imx8mn: use imx8m_clk_hw_composite_core

Use imx8m_clk_hw_composite_core to simplify code.

Add new definitions, and X_SRC/CG/DIV will be alias to the new
definitions for backwards compatibility

Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>