dma: sdma: Add imx25 compatible

imx25 did not work without a firmware previously.

This patch adds a DT compatible to pass the correct data with the
default script addresses for imx25.

Add imx25 compatible to the list of compatibles in the binding
documentation.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>