Add the required 'mboxes' property to the C71x DSP processor for the TI
J721E common processor board. The mailboxes and some shared memory are
required for running the Remote Processor Messaging (RPMsg) stack between
the host processor and each of the DSPs. The nodes are therefore added
in the common k3-j721e-som-p0.dtsi file so that all of these can be
co-located.

The chosen sub-mailboxes match the values used in the current firmware
images. This can be changed, if needed, as per the system integration
needs after making appropriate changes on the firmware side as well.
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20200825172145.13186-7-s-anna@ti.com