MMC: OMAP: Power functions modified to MMC multislot support

Modifications at power functions to MMC multislot support. This patch
also move board-specific code out of MMC OMAP driver.

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>