omap_hsmmc: Implement scatter-gather emulation

Instead of using the bounce buffer, using scatter-gather emulation
(as in the OMAP1/2 MMC driver) removes the need of one extra memory
copy and improves performance.

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>