arm: mx51: define mx51's own MXC_GPIO_IRQS

mx5 SoCs have different GPIO port count.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>