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>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1591714640-10332-3-git-send-email-tharvey@gateworks.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>