dt-bindings: mailbox: xlnx,zynqmp-ipi-mailbox: Document msg region requirement

dt-bindings: mailbox: xlnx,zynqmp-ipi-mailbox: Document msg region requirement

Add description clarifying that for Versal IPI mailboxes, both host and
remote agents must have the "msg" register region defined for successful
message passing. Without both, only notification-based communication
works.

Signed-off-by: Harini T <harini.t@amd.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>