MX2: Add DMA support for mx2 and (eventually) mx1

This patch adds DMA support for Freescale i.MX27 SoCs. It is derived
from the i.MX1 port and should (though currently untested) still be
working for the i.MX1.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>