arm64: dts: qcom: sc8280xp-x13s: mark s12b regulator as always-on

The s12b supply is used by several pmic regulators as well as the
wlan/bluetooth radio which are not yet fully described in the
devicetree.

Mark the regulator as always-on for now.

Fixes: f29077d86652 ("arm64: dts: qcom: sc8280xp-x13s: Add soundcard support")
Cc: Srinivas Kandagatla <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Bjorn Andersson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/arm64/boot/dts/qcom/sc8280xp-lenovo-thinkpad-x13s.dts

···
 			regulator-min-microvolt = <984000>;
 			regulator-max-microvolt = <984000>;
 			regulator-initial-mode = <RPMH_REGULATOR_MODE_HPM>;
+			regulator-always-on;
 		};
 
 		vreg_l3b: ldo3 {