extcon: arizona: Update binding docs to mention new defines for GPSW

Update the binding documentation to mention the defines added for the
possible values of wlf,gpsw.
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

Documentation/devicetree/bindings/extcon/extcon-arizona.txt

@@ -46,7 +46,8 @@ Optional properties:
     The second cell represents the MICBIAS to be used.
     The third cell represents the value of the micd-pol-gpio pin.
 
-  - wlf,gpsw : Settings for the general purpose switch
+  - wlf,gpsw : Settings for the general purpose switch, set as one of the
+    ARIZONA_GPSW_XXX defines.
 
 Example: