dt-bindings: mailbox: omap: Update bindings 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. Update the existing OMAP Mailbox bindings for this new IP present
on TI K3 AM65x and J721E SoCs. The same compatible from AM65x SoCs is
reused for J721E SoCs.

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