arm64: dts: qcom: apq8016-sbc: Fix 1.8V power rail on LS expansion

The 96Boards specification expects a 1.8V power rail on the low-speed
expansion connector that is able to provide at least 0.18W / 100 mA.
According to the DB410c hardware user manual this is done by connecting
both L15 and L16 in parallel with up to 55mA each (for 110 mA total) [1].

Unfortunately the current regulator setup in the DB410c device tree
does not implement the specification correctly and only provides 5 mA:

  - Only L15 is marked always-on, so L16 is never enabled.
  - Without specifying a load the regulator is put into LPM where
    it can only provide 5 mA.

Fix this by:

  - Adding proper voltage constraints for L16.
  - Making L16 always-on.
  - Adding regulator-system-load for both L15 and L16. 100 mA should be
    available in total, so specify 50 mA for each. (The regulator
    hardware can only be in normal (55 mA) or low-power mode (5 mA) so
    this will actually result in the expected 110 mA total...)

[1]: https://www.96boards.org/documentation/consumer/dragonboard/dragonboard410c/hardware-docs/hardware-user-manual.md.html#power-supplies

Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Fixes: 828dd5d6 ("arm64: dts: apq8016-sbc: make 1.8v available on LS expansion")
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230510-msm8916-regulators-v1-2-54d4960a05fc@gerhold.net

arch/arm64/boot/dts/qcom/apq8016-sbc.dts

@@ -526,19 +526,27 @@ l14 {
 		regulator-max-microvolt = <3300000>;
 	};
 
-	/**
-	 * 1.8v required on LS expansion
-	 * for mezzanine boards
+	/*
+	 * The 96Boards specification expects a 1.8V power rail on the low-speed
+	 * expansion connector that is able to provide at least 0.18W / 100 mA.
+	 * L15/L16 are connected in parallel to provide 55 mA each. A minimum load
+	 * must be specified to ensure the regulators are not put in LPM where they
+	 * would only provide 5 mA.
 	 */
 	l15 {
 		regulator-min-microvolt = <1800000>;
 		regulator-max-microvolt = <1800000>;
+		regulator-system-load = <50000>;
+		regulator-allow-set-load;
 		regulator-always-on;
 	};
 
 	l16 {
 		regulator-min-microvolt = <1800000>;
-		regulator-max-microvolt = <3300000>;
+		regulator-max-microvolt = <1800000>;
+		regulator-system-load = <50000>;
+		regulator-allow-set-load;
+		regulator-always-on;
 	};
 
 	l17 {