arm: kirkwood: readynas duo v2: Add GMT G762 Fan Controller

Now that GMT G762 PWM Fan Controller is available upstream, let's add
missing bindings in Netgear ReadyNAS Duo v2 .dts file.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>