clk: zynqmp: Add a check for NULL pointer

Add a NULL pointer check as clk_hw_get_parent can return NULL.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Link: https://lore.kernel.org/r/20220518055314.2486-1-shubhrajyoti.datta@xilinx.com


Acked-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>