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: f29077d8 ("arm64: dts: qcom: sc8280xp-x13s: Add soundcard support")
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230322113318.17908-4-johan+linaro@kernel.org