soc: qcom: spm: Read the voltage value from PMIC_STS register

Currently msm_spm_get_vdd returns the value recently written to the PMIC
from SPM, but it does not reflect the current voltage at PMIC in case
AVS/CPR is enabled. Change this API to read the current voltage in the
PMIC_STS register and return this value.

Change-Id: Ie3c0ad395d2023555d360dc34efa140602cd4d24
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>