MMC: OMAP: Add back cover switch support

This patch adds back MMC cover switch support in a way that
supports multiple slots.

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>