ARM: imx: Change the way nand devices are registered (generic part)

Instead of having a platform device defined in a devices.c and register
it in the machine file allocate it dynamically.  This reduces the size
of needed memory as the new function is discarded after init and only
the existing devices are created.

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