irq/generic-chip: Export irq_init_generic_chip() locally

This function will be used in the devres variant of
irq_alloc_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/1496246820-13250-4-git-send-email-brgl@bgdev.pl