clk: imx: Add check for kcalloc

As the potential failure of the kcalloc(),
it should be better to check it in order to
avoid the dereference of the NULL pointer.

Fixes: 379c9a24cc23 ("clk: imx: Fix reparenting of UARTs not associated with stdout")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Link: https://lore.kernel.org/r/20220310080257.1988412-1-jiasheng@iscas.ac.cn
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
