imx: imx8m: fix boot when CONFIG_$(SPL_)CLK not defined

When CONFIG_$(SPL_)CLK not defined, the clock controller device
not exist, so to avoid boot failure for platform not have
CONFIG_$(SPL_)CLK, add a check.

Reviewed-by: Patrick Wildt <patrick@blueri.se>
Tested-by: Patrick Wildt <patrick@blueri.se>
Signed-off-by: Peng Fan <peng.fan@nxp.com>