Allows callers to install their own handlers for when a GPIO line
changes state (such as for hotplug detect).

This also fixes a bug where we weren't acknowledging the GPIO IRQ
until after the bottom half had run, causing a severe IRQ storm
in some cases.
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>