Input: ads7846: Add description how to use internal reference (ADS7846)

By simply setting vref-mv to 0 the internal reference will be used.
Document that!

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/input/ads7846.txt

@@ -29,6 +29,8 @@ Optional properties:
 	ti,vref-delay-usecs		vref supply delay in usecs, 0 for
 					external vref (u16).
 	ti,vref-mv			The VREF voltage, in millivolts (u16).
+					Set to 0 to use internal refernce
+					(ADS7846).
 	ti,keep-vref-on			set to keep vref on for differential
 					measurements as well
 	ti,swap-xy			swap x and y axis