gpiolib: add description for gpio irqchip fields in struct gpio_chip

Add missed description for GPIO irqchip fields in struct gpio_chip.
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/driver.h

@@ -65,6 +65,17 @@ struct seq_file;
  *	registers.
  * @irq_not_threaded: flag must be set if @can_sleep is set but the
  *	IRQs don't need to be threaded
+ * @irqchip: GPIO IRQ chip impl, provided by GPIO driver
+ * @irqdomain: Interrupt translation domain; responsible for mapping
+ *	between GPIO hwirq number and linux irq number
+ * @irq_base: first linux IRQ number assigned to GPIO IRQ chip (deprecated)
+ * @irq_handler: the irq handler to use (often a predefined irq core function)
+ *	for GPIO IRQs, provided by GPIO driver
+ * @irq_default_type: default IRQ triggering type applied during GPIO driver
+ *	initialization, provided by GPIO driver
+ * @irq_parent: GPIO IRQ chip parent/bank linux irq number,
+ *	provided by GPIO driver
+ * @lock_key: per GPIO IRQ chip lockdep class
  *
  * A gpio_chip can help platforms abstract various sources of GPIOs so
  * they can all be accessed through a common programing interface.