regulator: cpr-regulator: add initial voltage reference property support

Add support in the cpr-regulator driver for a new device tree
property named qcom,cpr-init-voltage-ref which specifies the
reference voltages to use when decoding initial voltage fuse
values.  This property decouples inital voltage fuse decoding
from the specification of CPR ceiling voltages.  This feature
is needed since some SoCs utilize reference voltages which
are different from the ceiling voltages.

Change-Id: I87cd8a053253fa538e773aefa774c53ab69063a2
Signed-off-by: David Collins <collinsd@codeaurora.org>