dt-bindings: spi: zynq: Convert Zynq QSPI binding to yaml

Convert spi-zynq-qspi.txt to yaml.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/4ece21a7e9691ed1e775fd6b0b4046b1562e44bd.1612951821.git.michal.simek@xilinx.com


Reviewed-by: Rob Herring <robh@kernel.org>