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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

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

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