ARM i.MX: get rid of wrong MXC_INTERNAL_IRQ usage

There are several occurences where MXC_INTERNAL_IRQ is
assumed to be the start of the gpio interrupts. It was never
meant this way. Replace these with gpio_to_irq.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Fabio Estevam <fabio.estevam@freescale.com>