MMC: OMAP: Introduce new multislot structure and change driver to use it

Introduce new MMC multislot structure and change driver to use it.

Note that MMC clocking is now enabled in mmc_omap_select_slot()
and disabled in mmc_omap_release_slot().

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Jarkko Lavinen <jarkko.lavinen@nokia.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>