arm/mx2: use per-SOC nand device

The previous commit introduced one nand device per SoC.  Use this
directly instead of the compatibility macro that will break for
multi-SoC kernels.  And while at it remove the compatibility macro now
that all in-tree users are fixed.

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