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: NDaniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: NDouglas Anderson <dianders@chromium.org>
Signed-off-by: NAlexandru Stan <amstan@chromium.org>
Link: https://lore.kernel.org/r/20201021220404.v3.2.Ie4d84af5a85e8dcb8f575845518fa39f324a827d@changeidSigned-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>

arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi

@@ -179,6 +179,15 @@
 	backlight: backlight {
 		compatible = "pwm-backlight";
 
+		/* The panels don't seem to like anything below ~ 5% */
+		brightness-levels = <
+			196 256 324 400 484 576 676 784 900 1024 1156 1296
+			1444 1600 1764 1936 2116 2304 2500 2704 2916 3136
+			3364 3600 3844 4096
+		>;
+		num-interpolated-steps = <64>;
+		default-brightness-level = <951>;
+
 		pwms = <&cros_ec_pwm 1>;
 		enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;
 		power-supply = <&ppvar_sys>;