dt-bindings: Add vendor-prefix for X-Powers

Signed-off-by: NCarlo Caione <carlo@caione.org>
Signed-off-by: NChen-Yu Tsai <wens@csie.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -193,6 +193,7 @@ voipac	Voipac Technologies s.r.o.
 winbond Winbond Electronics corp.
 wlf	Wolfson Microelectronics
 wm	Wondermedia Technologies, Inc.
+x-powers	X-Powers
 xes	Extreme Engineering Solutions (X-ES)
 xillybus	Xillybus Ltd.
 xlnx	Xilinx