i.MX: add support for lower and upper interrupt in GPIO.

The i.MX6 GPIO device supports 2 interrupts instead of one.

* 1 for the lower 16 GPIOs.
* 1 for the upper 16 GPIOs.

i.MX31 and i.MX25 only support 1 interrupt for the 32 GPIOs.

So we add a property to turn the behavior on when required.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1447497668-1603-1-git-send-email-jcd@tribudubois.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>