gpio: fix kernel-doc notation warning for 'request_key'

Fix kernel-doc warning for missing struct member 'request_key':

../include/linux/gpio/driver.h:142: warning: Function parameter or member 'request_key' not described in 'gpio_irq_chip'

Fixes: 39c3fd58 ("kernel/irq: Extend lockdep class for request mutex")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/driver.h

@@ -66,9 +66,15 @@ struct gpio_irq_chip {
 	/**
 	 * @lock_key:
 	 *
-	 * Per GPIO IRQ chip lockdep classes.
+	 * Per GPIO IRQ chip lockdep class for IRQ lock.
 	 */
 	struct lock_class_key *lock_key;
+
+	/**
+	 * @request_key:
+	 *
+	 * Per GPIO IRQ chip lockdep class for IRQ request.
+	 */
 	struct lock_class_key *request_key;
 
 	/**