ARM: OMAP3: Add support for configuring MMC pins as GPIO pins

Some devices are using the MMC1 pins 4..8 as GPIO pins, and in
this case they need to be configured for 1.8V IO voltage if not
done by the bootloader as otherwise some devices like smsc911x
won't work properly.

Let's also make sure this register is saved and restored for
idle.

Cc: Tim Nordell <tim.nordell@logicpd.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>