ARM: mxc: make imx_dma_is_general_purpose more generic for sdma

sdma device names vary for different SoC. So we just check
whether it includes "sdma" substring.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>