arm64: dts: ti: k3-j721e: Enable Mailbox nodes at the board level

arm64: dts: ti: k3-j721e: Enable Mailbox nodes at the board level

Mailbox nodes defined in the top-level J721e SoC dtsi files are incomplete
and may not be functional unless they are extended with a chosen interrupt
and connection to a remote processor.

As the remote processors depend on memory nodes which are only known at
the board integration level, these nodes should only be enabled when
provided with the above information.

Disable the Mailbox nodes in the dtsi files and only enable the ones that
are actually used on a given board.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
Link: https://lore.kernel.org/r/20221020160305.18711-9-afd@ti.com