dt-bindings: mailbox: zynqmp_ipi: fix unit address

Fix unit address to match the first address specified in the reg
property of the node in example.
Signed-off-by: Kangmin Park <l4stpr0gr4m@gmail.com>
Link: https://lore.kernel.org/r/20200625135158.5861-1-l4stpr0gr4m@gmail.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/mailbox/xlnx,zynqmp-ipi-mailbox.txt

@@ -87,7 +87,7 @@ Example:
 		ranges;
 
 		/* APU<->RPU0 IPI mailbox controller */
-		ipi_mailbox_rpu0: mailbox@ff90400 {
+		ipi_mailbox_rpu0: mailbox@ff990400 {
 			reg = <0xff990400 0x20>,
 			      <0xff990420 0x20>,
 			      <0xff990080 0x20>,