regulator: cpr3-regulator: add support for open-loop voltage min difference

Add support for the qcom,cpr-open-loop-voltage-min-diff device
tree property which specifies the minimum allowed open-loop
voltage difference between adjacent CPR voltage corners.  This
property can be used to specifically allow open-loop voltage
monotonicity to be violated for selected corners.

Change-Id: Ib3c20ff882326a3550827f8acc1ac844530e60ba
Signed-off-by: David Collins <collinsd@codeaurora.org>