mmc: mxcmmc: enable DMA support on mpc512x

Add SDHC DMA channel description to the mpc512x device tree to enable
slave channel requesting in the mxcmmc driver.

mpc512x DMA engine doesn't support endianness conversion when
reading/writing data from peripheral's FIFO, so we have to swap data
buffers before each DMA write and after each DMA read transfer manually.

Since chained SDHC DMA transfers are not supported on mpc512x, limit
'max_segs' tunable parameter to one and initialise it to 64 only when
running on i.MX platforms.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>