dt-bindings: spi: Add device tree binding documentation for Zynq QSPI controller

This patch adds the dts binding document for Zynq SOC QSPI controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Naga Sureshkumar Relli <naga.sureshkumar.relli@xilinx.com>
Signed-off-by: Mark Brown <broonie@kernel.org>