dt-bindings: mailbox: add Versal IPI bindings

Add documentation for AMD-Xilinx Versal platform Inter Processor Interrupt
controller. Versal IPI controller contains buffer-less IPI which do not
have buffers for message passing. For such IPI channels message buffers
are not expected and only notification to/from remote agent is expected.

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>