arm64: dts: ti: iot2050: Add/enabled mailboxes and carve-outs for R5F cores

Analogously to the am654-base-board, configure the mailboxes for the two
R5F cores, add them and the already existing memory carve-outs to the
related MCU nodes. Allows to load applications under Linux onto the
cores, e.g. the RTI watchdog firmware.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/1776f8be19b39a938d9248fcfc5332b753783c3e.1632657917.git.jan.kiszka@web.de