backlight: Add support for Arctic Sand LED backlight driver chips

This driver provides support for the Arctic Sand arc2c0608 chip,
and provides a framework to support future devices.

Signed-off-by: Olimpiu Dejeu <olimpiu@arcticsand.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>