mailbox: omap: Add support for TI K3 SoCs

The TI K3 AM65x and J721E family of SoCs have a new Mailbox IP that
is based on the existing Mailbox IP present in OMAP architecture based
SoCs. Each instance of the legacy OMAP Mailbox IP is now a single cluster
within the newer Mailbox IP instance on K3 architecture based SoCs. A
single K3 Mailbox IP instance has multiple clusters with each cluster
providing the same functionality as the existing OMAP Mailbox IP.

Reuse the existing OMAP Mailbox driver to extend the support for this
newer IP present within the Main NavSS block on K3 SoCs. The K3 family
of SoCs use 64-bit ARMv8 processors for running Linux, so the driver is
also enhanced to deal with the differences between the 32-bit message
payloads and the 64-bit pointers used by the client drivers.

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