gpio: Document device_node's det_debounce

This patch adds documentation for set_debounce in struct device_node.
Signed-off-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/asm-generic/gpio.h

@@ -60,6 +60,8 @@ struct device_node;
  * @get: returns value for signal "offset"; for output signals this
  *	returns either the value actually sensed, or zero
  * @direction_output: configures signal "offset" as output, or returns error
+ * @set_debounce: optional hook for setting debounce time for specified gpio in
+ *      interrupt triggered gpio chips
  * @set: assigns output value for signal "offset"
  * @to_irq: optional hook supporting non-static gpio_to_irq() mappings;
  *	implementation may not sleep