devicetree: Document Wolfson prefix

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -37,4 +37,5 @@ simtek
 sirf	SiRF Technology, Inc.
 stericsson	ST-Ericsson
 ti	Texas Instruments
+wlf	Wolfson Microelectronics
 xlnx	Xilinx