dt-bindings: mailbox: omap: Update binding for AM64x SoCs

Update the existing OMAP Mailbox binding to include the info for
AM64x SoCs. There are some minor IP integration differences between
the AM64x SoCs and the previous AM65x and J721E SoC families.

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