drivers: mbox: Add support for TI OMAP mailbox

TI OMAP mailbox is the inter-processor mailbox IP found in TI
K3 devices (AM62X, AM64X, J721E .etc). The mailbox hardware uses
a queued mailbox interrupt mechanism that provides a communication
channel between processors through a set of registers and their
associated interrupt signals by sending and receiving messages.
The interrupt/bank associated with each processor entity is found
through the  usr_id property from device tree.

Signed-off-by: Vaishnav Achath <vaishnav.a@ti.com>