clk: imx8mp: use imx8m_clk_hw_composite_core to simplify code

Use imx8m_clk_hw_composite_core to simpliy clks that belong to
core clk slice.

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>