power: reset: linkstation-poweroff: add new device

This commit introduces support for NETGEAR ReadyNAS Duo v2.
This device use bit 4 of LED[2:0] Polarity Control Register to indicate
AC Power loss.

For more details about AC loss detection in NETGEAR ReadyNAS Duo v2,
please look at the file:
RND_5.3.13_WW.src/u-boot/board/mv_feroceon/mv_hal/usibootup/usibootup.c
from Netgear GPL sources.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>