ARM: OMAP2: delete board-rm680

Delete board file for Nokia RM-680/RM-696 (N950/N9). DT-based booting
should be used for further development on this HW.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>