mmc: mxs-mmc: have dma_channel than dma_res in mxs_mmc_host

It replaces dma_res with dma_channel in struct mxs_mmc_host, so that
the device tree support will be a little easier, since dma channel
can not be retrieved from "struct resource *dma_res".

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Chris Ball <cjb@laptop.org>