ARM: dts: DRA7: Add mailbox nodes

DRA7xx has 13 system mailboxes, and is present on both the
DRA72x and DRA74x family of SoCs. Add the DT nodes for all
these 13 mailboxes. Except for mailbox 1, all other mailboxes
do not have interrupts mapped into the MPU GIC by default.

All the mailboxes have been disabled and the interrupts
property information is left out intentionally for now,
because of the dependencies against the crossbar driver.
These mailboxes can be enabled when a usecase arises
and the crossbar driver dependencies are met.

NOTE: The mailbox 1 has different number of mailbox fifos
and IP interrupts compared to the remaining 12 mailboxes.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>