MMC: OMAP: New release dma and abort xfer functions

New functions to support MMC multislot:
mmc_omap_release_dma() and mmc_omap_abort_xfer().

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Carlos Eduardo Aguiar <carlos.aguiar@indt.org.br>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>