backlight: pwm_bl: Set scale type for brightness curves specified in the DT

Check if a brightness curve specified in the device tree is linear or
not and set the corresponding property accordingly. This makes the
scale type available to userspace via the 'scale' sysfs attribute.

To determine if a curve is linear it is compared to a interpolated linear
curve between min and max brightness. The curve is considered linear if
no value deviates more than +/-5% of ${brightness_range} from their
interpolated value.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>