of: add vendor prefix for Allwinner Technology

We have been using the "allwinner" prefix for everything so far; let's
document it here.

Signed-off-by: Emilio López <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/vendor-prefixes.txt

···
 adi	Analog Devices, Inc.
 aeroflexgaisler	Aeroflex Gaisler AB
 ak	Asahi Kasei Corp.
+allwinner	Allwinner Technology Co., Ltd.
 altr	Altera Corp.
 amcc	Applied Micro Circuits Corporation (APM, formally AMCC)
 amstaos	AMS-Taos Inc.