drivers: Defer probe if firmware is not ready

Driver needs ZynqMP firmware interface to call EEMI
APIs. In case firmware is not ready, dependent drivers
should wait until the firmware is ready.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

include/linux/firmware/xlnx-zynqmp.h

@@ -293,7 +293,7 @@ const struct zynqmp_eemi_ops *zynqmp_pm_get_eemi_ops(void);
 #else
 static inline struct zynqmp_eemi_ops *zynqmp_pm_get_eemi_ops(void)
 {
-	return NULL;
+	return ERR_PTR(-ENODEV);
 }
 #endif