dt-bindings: mailbox: zynqmp_ipi: convert to yaml

Convert the ipi doc to yaml.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220517165536.8637-1-shubhrajyoti.datta@xilinx.com