mmc: atmel-mci: restore dma on AVR32

Commit ecb89f2f ("mmc: atmel-mci: remove compat for non DT board
when requesting dma chan") broke dma on AVR32 and any other boards not
using DT.  This restores a fallback mechanism for such cases.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>