tifm_sd: implement software scatter-gather

It was found that delays associated with issue and completion of the commands
severely limit performance of the new, fast SD cards. To alleviate this issue
scatter-gather emulation in software is implemented for both dma and pio
transfer modes. Non-block aligned and high memory sg entries are accounted
for.

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>