arm64: dts: ti: k3-am65-base-board: Add IPC sub-mailbox nodes for R5Fs

Add the sub-mailbox nodes that are used to communicate between MPU and
the two R5F remote processors present in the MCU domain to the AM654
EVM base board. These sub-mailbox nodes utilize the System Mailbox
clusters 0 and 1. The interrupts associated with the Mailbox Cluster
User interrupt used by the sub-mailbox nodes are also added. The GIC_SPI
interrupt to be used is dynamically allocated and managed by the System
Firmware through the ti-sci-intr irqchip driver. All the remaining
mailbox clusters are currently not used on A53 core, and so are disabled.

The sub-mailbox nodes added match the hard-coded mailbox configuration
used within the TI RTOS IPC software packages. The Cortex R5F processor
sub-system is assumed to be running in Split mode, so a sub-mailbox node
is used by each of the R5F cores. Only the sub-mailbox node from cluster 0
is used in case of Lockstep mode.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>