arm/mx2: define seperate gpio port descriptions for imx21 and imx27

As the gpio ports have different addresses on imx21 and imx27 there are
two different port descriptions needed if not relying on the overloaded
cpp macro IO_ADDRESS.  So some cpp magic is added to minimize code
duplication.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>