arm64: dts: ti: k3-j721e-beagleboneai64: Fix mailbox node status

arm64: dts: ti: k3-j721e-beagleboneai64: Fix mailbox node status

Mailbox nodes are now disabled by default. The BeagleBoard AI64 DT
addition went in at around the same time and must have missed that
change so the mailboxes are not re-enabled. Do that here.

Fixes: fae14a1cb8dd ("arm64: dts: ti: Add k3-j721e-beagleboneai64")
Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20230515172137.474626-1-afd@ti.com
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>