mmc: mmci: Enforce DMA configuration through DT

Remove the option to provide DMA configuration as platform data,
enforce it through DT.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Roland Stigge <stigge@antcom.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>