mfd: stmpe: Document DT binding for irq_over_gpio

STMPE now supports using a GPIO as an IRQ source.  Document the device
tree binding for this option.

Signed-off-by: Sean Cross <xobs@kosagi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/stmpe.txt

@@ -13,6 +13,7 @@ Optional properties:
  - interrupt-parent             : Specifies which IRQ controller we're connected to
  - wakeup-source                : Marks the input device as wakable
  - st,autosleep-timeout         : Valid entries (ms); 4, 16, 32, 64, 128, 256, 512 and 1024
+ - irq-gpio                     : If present, which GPIO to use for event IRQ
 
 Example: