ARM: dts: imx: Move SDMA firmware to SoC dtsi file

Instead of adding sdma-ram-script-name into each board dts file, move
it to the SoC dtsi file instead.

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