MA-17597 clk: imx8mp: remove __init in imx_clk_init_on

MA-17597 clk: imx8mp: remove __init in imx_clk_init_on

Fix below section mismatch build warnings:
  The function imx8mp_clocks_probe() references
  the function __init imx_clk_init_on().
  This is often because imx8mp_clocks_probe lacks a __init
  annotation or the annotation of imx_clk_init_on is wrong.

  FATAL: modpost: Section mismatches detected.
  Set CONFIG_SECTION_MISMATCH_WARN_ONLY=y to allow them.

Reviewed-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Jindong Yue <jindong.yue@nxp.com>