dt-bindings: mailbox: zynqmp: extend required list

"xlnx,ipi-id" is handled as required property but is
missing from binding doc required list of mailbox child node.
Add that to required list. This does not break backward
compatibility but bug in bindings document.

Fixes: 4a855a95

 ("dt-bindings: mailbox: zynqmp_ipi: convert to yaml")
Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>