arm64: dts: qcom: trogdor: Add brightness-levels

arm64: dts: qcom: trogdor: Add brightness-levels

We want userspace to represent the human perceived brightness.
Since the led drivers and the leds themselves don't have a
linear response to the value we give them in terms of perceived
brightness, we'll bake the curve into the dts.

The panel also doesn't have a good response under 5%, so we'll avoid
sending it anything lower than that.

Note: Ideally this patch should be coupled with the driver change from
"backlight: pwm_bl: Fix interpolation", but it can work without it,
without looking too ugly.

Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Alexandru Stan <amstan@chromium.org>
Link: https://lore.kernel.org/r/20201021220404.v3.2.Ie4d84af5a85e8dcb8f575845518fa39f324a827d@changeid
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>