dmaengine i.MX SDMA: Allow to run without firmware

The SDMA firmware consists of a ROM part and a RAM part.
The ROM part is always present in the SDMA engine and
is sufficient for many cases.
This patch allows to pass in platform data containing
the script addresses in ROM, so loading a firmware is
optional now.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Dan Williams <dan.j.williams@intel.com>