ARM: dts: omap3-devkit8000: Add user button support

This patch links the user button to the BTN_EXTRA action.

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
[tony@atomide.com: added missing sign as noted by Anthoine]
Signed-off-by: Tony Lindgren <tony@atomide.com>