drivers: regulator: qcom: add PMS405 SPMI regulator

The PMS405 sports 5 HFSMPS and 13 LDO regulators,

This commit adds support for one of the 5 HFSMPS regulators (s3) to
the spmi regulator driver.

The PMIC HFSMPS 430 regulators have 8 mV step size and a voltage
control scheme consisting of two  8-bit registers defining a 16-bit
voltage set point in units of millivolts

S3 controls the cpu voltages (s3 is a buck regulator of type HFS430);
it is therefore required so we can enable voltage scaling for safely
running cpufreq.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>