mailbox: ti-msgmgr: Move the memory region name to descriptor

For newer generation of the hardware, the naming of the region is
decided at integration level and there could be additional regions
as well. Hence move the region naming to be described from compatible
descriptor.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>