dt-bindings: mfd: da9052: Support TSI as ADC

DA9052 allows using the touchscreen input pins as general purpose
analogue input pin by wiring analogue inputs to X+, X-, Y+ and Y-
and providing a reference voltage at TSIREF pin.
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/da9052-i2c.txt

@@ -4,6 +4,14 @@ Required properties:
 - compatible : Should be "dlg,da9052", "dlg,da9053-aa",
 			 "dlg,da9053-ab", or "dlg,da9053-bb"
 
+Optional properties:
+- dlg,tsi-as-adc : Boolean, if set the X+, X-, Y+, Y- touchscreen
+                    input lines are used as general purpose analogue
+					input.
+- tsiref-supply: Phandle to the regulator, which provides the reference
+                 voltage for the TSIREF pin. Must be provided when the
+			     touchscreen pins are used for ADC purposes.
+
 Sub-nodes:
 - regulators : Contain the regulator nodes. The DA9052/53 regulators are
   bound using their names as listed below: