ARM: dts: imx6qdl-gw: add Gateworks System Controller support

Add Gateworks System Controller support to Gateworks Ventana boards:
- add dt bindings for GSC mfd driver and hwmon driver for ADC's and
  fan controllers.
- add dt bindings for gpio-keys driver for push-button and interrupt events

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>