power: battery_current_limit: Support Vph battery voltage monitoring

Add support to monitor the Vph battery voltage.

When enabled, BCL driver sets thresholds for the Vph battery
voltage monitoring. When the Vph voltage goes below a certain
threshold, it enables the Ibat current monitoring. When the
Vph voltage goes above a certain high threshold, BCL driver
disables the Ibat monitoring and any CPU frequency mitigation.

BCL driver supports two new sysfs nodes vph_high_thresh_uv and
vph_low_thresh_uv to configure the Vph high and low thresholds
respectively. These values can be configured by disabling the
BCL driver and once done with the configuring the BCL driver
should be re-enabled to set the new configuration.

BCL driver exposes two API's, which can be used by other
battery monitoring driver to notify the battery voltage
and current threshold status. Based on these status
notifications, BCL driver will apply or remove the
CPU mitigation.

CRs-Fixed: 629894
Change-Id: Ia45079e3a49387802b3bc1690b40919670824477
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>