MFC r343952, r344003, r344219, r344343, r344456

r343952 by ganbold:
Enable necessary bits when activating interrupts. This allows
reading some events from the interrupt status registers. These events
are reported to devd via system "PMU" and subsystem "Battery", "AC"
and "USB" such as plugged/unplugged, absent, charged and charging.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D19116

r344003 by ganbold:
Add sensors support for AXP803/AXP813. Sensor values such as
battery charging, charge state, voltage, charging current, discharging current,
battery capacity etc. can be obtained via sysctl.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D19145

r344219 by ganbold:
Add sysctl for setting battery charging current.
The charging current can be set using steps
from 0: 200mA to 13: 2800mA (200mA/step).
While there, fix battery charging current related
sensor descriptions.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D19212

r344343 by ganbold:
Clarify notifications when battery capacity ratio
reaches warning and shutdown thresholds.

r344456 by ganbold:
Add base to the warning threshold.