As a side effect this makes IMX_DMA selectable on i.MX21 again, because
the symbol ARCH_MX21 doesn't exist (MACH_MX21 would have been more correct).
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>