orion/kirkwood: create a generic function for gpio led blinking

dns323 and (at least) iconnect platforms are using hw led blinking, so,
instead of having 2 identicals .gpio_blink_set gpio-led hooks, move
dns323 code into gpio.c

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Tested-By: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>