dt-bindings: mfd: gateworks-gsc: Add fan-tach mode

In 7497d4a6 ("hwmon: (gsc-hwmon) add fan sensor") a mode
was added to report RPM's from a fan tach input.

Add this mode to the dt-bindings for the Gateworks System Controller.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>