omap3beagle: add a platform device to hook up the GPIO leds to the leds-gpio driver

omap3beagle: add a platform device to hook up the GPIO leds to the leds-gpio driver
 * on revision A5 and earlier board the two leds can't be controlled seperately,
   should be fixed in rev. B and C boards.

Changes since v1: indented structs with tabs

Signed-off-by: Koen Kooi <koen@openembedded.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>