irq/generic-chip: Provide irq_free_generic_chip()

Currently there's no way for users of irq_alloc_generic_chip() to free
the allocated memory other than calling kfree() manually on the
returned pointer. This may lead to errors if the internals of
irq_alloc_generic_chip() ever change. Provide a routine to free the
generic chip.

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/1496246820-13250-2-git-send-email-brgl@bgdev.pl