power: avs: Add support for CPR (Core Power Reduction)

CPR (Core Power Reduction) is a technology that reduces core power on a
CPU or other device. It reads voltage settings in efuse from product
test process as initial settings.
Each OPP corresponds to a "corner" that has a range of valid voltages
for a particular frequency. While the device is running at a particular
frequency, CPR monitors dynamic factors such as temperature, etc. and
adjusts the voltage for that frequency accordingly to save power
and meet silicon characteristic requirements.

This driver is based on an RFC by Stephen Boyd[1], which in turn is
based on work by others on codeaurora.org[2].

[1] https://lkml.org/lkml/2015/9/18/833
[2] https://www.codeaurora.org/cgit/quic/la/kernel/msm-3.10/tree/drivers/regulator/cpr-regulator.c?h=msm-3.10



Changes since RFC:
-Added support for debugfs.
-Verify that APC regulator supports the range required by each OPP.
-Updated MAINTAINERS file.

Co-developed-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>