Current dt-bindings does not contain compatible property
for child mailbox node. Child mailbox nodes are used
to represent destination (remote) IPI agents. The compatible
property for all destination mailboxes must be required to
identify the node.

This addition of new required property does not break ABI
as current driver does not use this compatible property
but, new features in driver should use this property.
Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>