arm64: dts: ti: k3-am65-main: Rename IPC sub-mailboxes

The name "mcu" is no longer to be used to represent the R5F processor
subsystem, and is being replaced with the name "r5fss". It will continue
to be used as a prefix for peripherals that are present within the MCU
voltage domain on the TI K3 SoCs.

Rename the current sub-mailbox nodes that are used to communicate
between the MPU and the two R5F remote processors in the MCU domain
accordingly. The name ipc3x is also dropped to keep the node names
generic and be agnostic of the RTOS IPC product to be used for future
scalability.

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

arch/arm64/boot/dts/ti/k3-am65-main.dtsi

@@ -307,7 +307,7 @@
 		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>;
 		};
@@ -322,7 +322,7 @@
 		interrupt-parent = <&main_navss_intr>;
 		interrupts = <165 0 IRQ_TYPE_LEVEL_HIGH>;
 
-		mbox_mcu_r5f1_ipc3x: mbox-mcu-r5f1-ipc3x {
+		mbox_mcu_r5fss0_core1: mbox-mcu-r5fss0-core1 {
 			ti,mbox-tx = <1 0 0>;
 			ti,mbox-rx = <0 0 0>;
 		};