regulator: qcom_spmi: Add support for S4 supply on pm8941

The S4 supply is sometimes called the boost regulator because it
outputs 5V. Typically it's connected to the 5vs1 and 5vs2
switches for use in USB OTG and HDMI applications. Add support
for this regulator which was mistakenly left out from the initial
submission of this driver.

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Fixes: e92a4047 ("regulator: Add QCOM SPMI regulator driver")
Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>