drivers: soc: qcom: spm: Add support for 16bit vlevel

SPM regulator driver can send 16bit voltage levels to be sent to
pmic arbiter for certain PMICs. SAW VCTL can have max pmic data
width as 8bit.

Update spm driver to send lower 8bit of voltage level first and
then send upper 8bit to PVC port. Voltage change happens when
upper 8bit is written to PMIC registers.

Change-Id: Iff1d1976adfe75058f6c64d4212c78d88422278c
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>
Signed-off-by: Srinivas Rao L <lsrao@codeaurora.org>
Signed-off-by: Chinkit Kumar,Kirti Kumar Parmar <parma@codeaurora.org>