arm64: zynqmp: Add missing destination mailbox compatible

The commit 81186dc1 ("dt-bindings: zynqmp: add destination mailbox
compatible") make compatible string for child nodes mandatory that's why
add it.
Signed-off-by: Michal Simek <michal.simek@amd.com>

arch/arm64/boot/dts/xilinx/zynqmp.dtsi

@@ -141,6 +141,7 @@ zynqmp_ipi: zynqmp-ipi {
 
 		ipi_mailbox_pmu1: mailbox@ff9905c0 {
 			bootph-all;
+			compatible = "xlnx,zynqmp-ipi-dest-mailbox";
 			reg = <0x0 0xff9905c0 0x0 0x20>,
 			      <0x0 0xff9905e0 0x0 0x20>,
 			      <0x0 0xff990e80 0x0 0x20>,