Input: atmel_mxt_ts - improve description of gpio-keymap property

The below patch improves the documentation for the gpio-property. Stephen
Warren has a good example here:
https://github.com/swarren/linux-tegra/commit/09789801

trackpad@4b {
  compatible = "atmel,maxtouch";
  reg = <0x4b>;
  interrupt-parent = <&gpio>;
  interrupts = <TEGRA_GPIO(W, 3) IRQ_TYPE_LEVEL_LOW>;
  linux,gpio-keymap = <0 0 0 BTN_LEFT>;
};

This maps BTN_LEFT to the 4th bit of the T19 message.
Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/atmel,maxtouch.txt

@@ -15,6 +15,17 @@ Optional properties for main touchpad device:
     keycode generated by each GPIO. Linux keycodes are defined in
     <dt-bindings/input/input.h>.
 
+- linux,gpio-keymap: When enabled, the SPT_GPIOPWN_T19 object sends messages
+    on GPIO bit changes. An array of up to 8 entries can be provided
+    indicating the Linux keycode mapped to each bit of the status byte,
+    starting at the LSB. Linux keycodes are defined in
+    <dt-bindings/input/input.h>.
+
+    Note: the numbering of the GPIOs and the bit they start at varies between
+    maXTouch devices. You must either refer to the documentation, or
+    experiment to determine which bit corresponds to which input. Use
+    KEY_RESERVED for unused padding values.
+
 Example:
 
 	touch@4b {