ARM: Kirkwood: enable GPIO fan alarm support for 2Big Network v2

On the LaCie 2Big Network v2 (net2big_v2) board, the fan alarm is not
wired to the I2C fan controller but to a separe GPIO. This GPIO can be
controlled by using the gpio-fan driver.

This patch adds the gpio-fan alarm description in the net2big_v2 DTS.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>