spi/xtensa-xtfpga: add DT binding documentation

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/spi/spi-xtensa-xtfpga.txt

@@ -0,0 +1,9 @@
+Cadence Xtensa XTFPGA platform SPI controller.
+
+This simple SPI master controller is built into xtfpga bitstreams and is used
+to control daughterboard audio codec.
+
+Required properties:
+- compatible: should be "cdns,xtfpga-spi".
+- reg: physical base address of the controller and length of memory mapped
+  region.