board: sam9x60-curiosity: Let LED subsystem init leds if enabled

If CONFIG_LED and CONFIG_LED_GPIO are enabled, it is not necessary to
initialize the RGB LED on the board by manually setting hardcoded GPIOs
anymore.  Everything is well defined in dts and can be used like on
boards of other vendors.

Keep the old behaviour as fallback, though.

With all this in place enabling CONFIG_CMD_LED gives us a working 'led'
command on the U-Boot shell.

Signed-off-by: Alexander Dahl <ada@thorsis.com>