i.MX51: handle IRQ for gpio 16..31

The i.MX51 generates 2 IRQ for each GPIO bank : one for gpio 0 to 15
and one for gpio 16 to 31.
Actually only the lower IRQ is registered so register the second one.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>