ARM: imx: new Kconfig symbol and feature test macro for DMA on mx1 and mx2

This should be used instead of hard coding the corresponding platforms.
The feature test macro is needed to support different SOCs in a single
kernel image.  While at it rename dma-mx1-mx2 to dma-v1 as mx25 doesn't
use it and so the mx2 part is wrong and move the header to
arch/arm/mach-imx.

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