mmc: atmel-mci: convert to dma_request_slave_channel_compat()

Use generic DMA DT helper. Platforms booting with or without DT populated
are both supported.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>