fix(zynqmp): resolve null pointer dereferencing

The upstream coverity tool has reported the null pointer dereferences
(NULL_RETURNS) warning.

The coverity warning,
Dereferencing a pointer that might be "NULL" "proc" when calling
"pm_client_suspend".

Fix the same by checking the NULL before processing further.

Change-Id: I33acead9250bab0ed24b94aa1c1bdc31e80de771
Signed-off-by: Ronak Jain <ronak.jain@amd.com>