dma: imx-sdma: clarify about firmware not found error

When a firmware cannot be found for the SDMA engine then we can
continue with the internal ROM firmware.

The meaning of this message is frequently asked for, so make clear
that the driver still works with the internal ROM firmware and reduce
the loglevel from err to info.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>