dt-bindings: mfd: gateworks-gsc: Add 16bit pre-scaled voltage mode

Add a 16-bit pre-scaled voltage mode to ADC and clarify that existing
pre-scaled mode is 24bit.

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