arm/mx2: define seperate imx_nand devices for imx21 and imx27

As the NFC controller has different addresses on imx21 and imx27 there
are two different devices needed if not relying on the overloaded cpp
macro NFC_BASE_ADDR.  So some cpp magic is added to minimize code
duplication.

As obviously these two defines need different names, the name of the old
device is #defined to the new one when building for only one of imx21 or
imx27.

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