mxc/iomux: add GPIO bank offset for iomux v3 platforms

These GPIO bank offsets are useful when define a gpio number.
E.G. when GPIO PORTC pin 6 is used for irq request pin of external
expanding device, we can define it like:
  #define EXP_PARENT_IRQ_PIN (GPIO_PORTC + 6)

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>