zynqmp: Make MMIO write FW call synchronous

We must guarantee that writes have become effective before returning to
the caller. Hence, wait for PMUFW signaling completion of the FW call
before returning to the rich OS.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>

plat/xilinx/zynqmp/pm_service/pm_api_sys.c

@@ -476,7 +476,7 @@ enum pm_ret_status pm_mmio_write(uintptr_t address,
 
 	/* Send request to the PMU */
 	PM_PACK_PAYLOAD4(payload, PM_MMIO_WRITE, address, mask, value);
-	return pm_ipi_send(primary_proc, payload);
+	return pm_ipi_send_sync(primary_proc, payload, NULL);
 }
 
 /**