dt-bindings: mailbox: omap: Update example for TI K3 AM65x SoCs

Update the current example node for the K3 AM65x SoCs in the OMAP
Mailbox bindings to reflect the updated node names and labels in
the actual source code.

Signed-off-by: Suman Anna <s-anna@ti.com>

Documentation/devicetree/bindings/mailbox/omap-mailbox.txt

@@ -165,7 +165,7 @@ mailbox: mailbox@480c8000 {
 		interrupt-parent = <&main_navss_intr>;
 		interrupts = <164 0 IRQ_TYPE_LEVEL_HIGH>;
 
-		mbox_mcu_r5f0_ipc3x: mbox-mcu-r5f0-ipc3x {
+		mbox_mcu_r5fss0_core0: mbox-mcu-r5fss0-core0 {
 			ti,mbox-tx = <1 0 0>;
 			ti,mbox-rx = <0 0 0>;
 		};