Fix below section mismatch build warning in imx8mm/mn/mq clock driver:

WARNING: modpost: vmlinux.o(.text+0x667618): Section mismatch in reference
from the function imx8mm_clocks_probe() to the function .init.text:imx_clk_init_on.isra.0()
The function imx8mm_clocks_probe() references
the function __init imx_clk_init_on.isra.0().
This is often because imx8mm_clocks_probe lacks a __init
annotation or the annotation of imx_clk_init_on.isra.0 is wrong.

WARNING: modpost: vmlinux.o(.text+0x66a034): Section mismatch in reference
from the function imx8mn_clocks_probe() to the function .init.text:imx_clk_init_on.isra.0()
The function imx8mn_clocks_probe() references
the function __init imx_clk_init_on.isra.0().
This is often because imx8mn_clocks_probe lacks a __init
annotation or the annotation of imx_clk_init_on.isra.0 is wrong.

WARNING: modpost: vmlinux.o(.text+0x6713f8): Section mismatch in reference
from the function imx8mq_clocks_probe() to the function .init.text:imx_clk_init_on.isra.0()
The function imx8mq_clocks_probe() references
the function __init imx_clk_init_on.isra.0().
This is often because imx8mq_clocks_probe lacks a __init
annotation or the annotation of imx_clk_init_on.isra.0 is wrong.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Jason Liu <jason.hui.liu@nxp.com>