[AVR32] leds-gpio for stk1000

Add GPIO led support: J2 to either block of LEDs on the STK1000.

This uses the new LEDS_GPIO driver, and sets up a heartbeat trigger by
default ... either bright (!!) amber, or a more interesting purple.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>