omap3 beagle: input button

Export Beagle's "user" button through the input framework so it's
accessible using normal mechanisms.  This uses BTN_EXTRA for lack
of a more obvious choice.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>

diff --cc arch/arm/mach-omap2/board-omap3beagle.c
Simple merge