dt-bindings: power: reset: xilinx: Add bindings for ipi mailbox

Add IPI mailbox property and its example in xilinx zynqmp-power
documentation.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>