net2big_v2: initialize I2C fan at startup

This patch ensures minimal cooling for the net2big_v2 by automatically
starting the I2C fan (GMT G762) at low speed (2800 RPM).

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>