gpio: stp-xway: automatically drive GPHY leds on ar10 and grx390

Ar10 (xr300) has 3 and grx390 (xrx330) has 4 built-in GPHY. PHY LEDs are
connected via STP. STP is a peripheral controller used to drive external
shift register cascades. The hardware is able to allow the GPHY to drive
some GPIO of the cascade automatically.This patch allows for this on ar10
and grx390.

Tested on D-Link DWR-966 with OpenWRT.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>