Add the required 'mboxes' property to all the IPU and DSP remote
processors (IPU1, IPU2, DSP1 and DSP2) in the two available common
dtsi files - dra7-ipu-dsp-common and dra74-ipu-dsp-common dtsi files.
The latter file is for platforms having DRA74x/DRA76x/AM572x/AM574x
SoCs which do have a DSP2 processor in addition to the other common
remote processors. The common data is added to the former file, and
the DSP2 only data is added to the latter file.

The mailboxes are required for running the Remote Processor Messaging
(RPMsg) stack between the host processor and each of the remote
processors. Each of the remote processors uses a single sub-mailbox
node, the IPUs are assumed to be running in SMP-mode. 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: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>