xlnx: dt-bindings: Convert to json-schema

Merge series from Vincenzo Frascino <vincenzo.frascino@arm.com>:

This series converts the folling Xilinx device tree binding documentation:
 - xlnx,i2s
 - xlnx,audio-formatter
 - xlnx,spdif
to json-schema.

Note: These bindings are required for future work on the ARM Morello
Platforms device tree.