Add the sub-mailbox nodes that are used to communicate between MPU and
various remote processors present in the J7200 SoCs to the J7200 common
processor board. These include the R5F remote processors in the dual-R5F
clusters in the MCU domain (MCU_R5FSS0) and the MAIN domain (MAIN_R5FSS0).
These sub-mailbox nodes utilize the System Mailbox clusters 0 and 1. All
the remaining mailbox clusters are currently not used on A72 core, and
so are disabled. The nodes are added in the k3-j7200-som-p0.dtsi file
to co-locate these alongside future reserved-memory nodes required for
remoteprocs.

The sub-mailbox nodes added match the hard-coded mailbox configuration
used within the TI RTOS IPC software packages. A sub-mailbox node is added
for each of the R5F cores to accommodate the R5F processor sub-systems
running in Split mode. Only the sub-mailbox node for the first R5F core in
each cluster is used in case of 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 that are used by the sub-mailbox devices (each cluster's
User 0 IRQ output) are enabled. This is done to minimize the number of
NavSS Interrupt Router outputs utilized.
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Praneeth Bajjuri <praneeth@ti.com>
Link: https://lore.kernel.org/r/20201026232637.15681-4-s-anna@ti.com