regulator: pmic8901-regulator: Add support for PMIC8901 regulator

On 8x60 PMIC LDO/SMPS/LVS are controlled from the apps processor.
This driver talks directly to the PMIC over SSBI through the
PMIC8901 core driver. This driver utilizes the regulator framework
to enable, disable, and change regulator voltage levels.

Change-Id: I21c272a490a2526c2fde9fe3ef7590a57418269a
Signed-off-by: Bobby Crabtree <bobbyc@codeaurora.org>