Add the sub-mailbox nodes that are used to communicate between MPU and
various remote processors present in the J721E SoCs to the J721E common
processor board. These include the R5F remote processors in the dual-R5F
cluster (MCU_R5FSS0) in the MCU domain and the two dual-R5F clusters
(MAIN_R5FSS0 & MAIN_R5FSS1) in the MAIN domain; the two C66x DSP remote
processors and the single C71x DSP remote processor in the MAIN domain.
These sub-mailbox nodes utilize the System Mailbox clusters 0 through 4.
All the remaining mailbox clusters are currently not used on A72 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 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 Lockstep mode
for that R5F cluster.

NOTE:
The GIC_SPI interrupts to be used are dynamically allocated and managed
by the System Firmware through the ti-sci-intr irqchip driver. So, only
valid interrupts (each cluster's User 0 IRQ output) that are used by the
sub-mailbox devices are enabled. This is done to minimize the number of
NavSS Interrupt Router outputs utilized.
Signed-off-by: NSuman Anna <s-anna@ti.com>
Signed-off-by: NTero Kristo <t-kristo@ti.com>