mailbox: omap: Add support for K3 AM64x SoCs

mailbox: omap: Add support for K3 AM64x SoCs

The AM64x SoC contains a Mailbox IP instance with multiple clusters
in the MAIN domain, and is a variant of the IP on current AM65x and
J721E SoCs. The AM64x SoC has only 8 clusters with no interrupts
routed to the A53 core on the first 2 clusters. The interrupt outputs
from the IP do not go through any Interrupt Routers and are hard-wired
to each processor, with only couple of interrupts from each cluster
reaching the A53 core. The IP is also not built with the K3 safety
feature in hardware.

Add the support for this IP through a new compatible.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>