dts: zynqmp: add missing destination mailbox compatible

The commit in linux kernel 81186dc16101 ("dt-bindings: zynqmp: add
destination mailbox compatible") make compatible string for child nodes
mandatory that's why add it.

Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Link: https://lore.kernel.org/r/20231204215620.63334-5-tanmay.shah@amd.com


Signed-off-by: Michal Simek <michal.simek@amd.com>