leds: leds-gpio for ngw100

Add GPIO leds to the NGW100 platform and its defconfig.
Access through /sys/class/leds/{a,b,sys}/* files; one
defaults to a heartbeat.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>