arm64: dts: ti: k3-am642-sk: Add IPC sub-mailbox nodes

Add the sub-mailbox nodes that are used to communicate between MPU and
various remote processors present in the AM64x SoCs for the AM642 SK
board. These include the R5F remote processors in the two dual-R5F
clusters (MAIN_R5FSS0 & MAIN_R5FSS1) in the MAIN domain; and a M4
processor in the MCU safety island.

These sub-mailbox nodes utilize the System Mailbox clusters 2, 4 and 6.
The remaining clusters 3, 5 and 7 are currently not used, and so are
disabled. Clusters 0 and 1 were never added to the dts file as they do
not support interrupts towards the A53 core.

The sub-mailbox nodes added match the hard-coded mailbox configuration
used within the TI RTOS IPC software packages. The R5F processor
sub-systems are 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 for
the first R5F core in each cluster is used in case of a Single-CPU mode
for that R5F cluster. The nodes are all identical ...