clk: imx8m: migrate A53 clk root to use composite core

Migrate A53 clk root to use composite core clk type. It
will simplify code and make it easy to use composite
specific mux operation.

Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
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>