omap mmc: Add low-level initialization for hsmmc controller

Add low-level initialization for hsmmc controller. Merged into
this patch patch are various improvments and board support by
Grazvydas Ignotas and David Brownell.

Also change wire4 to be wires, as some newer controllers support
8 data lines.

Cc: Pierre Ossman <drzeus-mmc@drzeus.cx>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>