irq/generic-chip: Provide irq_destroy_generic_chip()

Most users of irq_alloc_generic_chip() call irq_setup_generic_chip()
too. To simplify the cleanup provide a function that both removes a
generic chip and frees its memory.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-doc@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Link: http://lkml.kernel.org/r/1496246820-13250-3-git-send-email-brgl@bgdev.pl