arm: omap: add minimal support for RM-680

Add minimal support for Nokia RM-680 board.

Tested with omap2plus_defconfig.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
[tony@atomide.com: updated to remove omap_gpio_init
Signed-off-by: Tony Lindgren <tony@atomide.com>