gpio: gpio-reg: add irq mapping for gpio-reg users

Add support for mapping gpio-reg gpios to interrupts.  This may be a
non-linear mapping - some gpios in the register may not even have
corresponding interrupts associated with them, so we need to pass an
array.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>