Pressing the PEK (power enable key) will shutdown the board.
Some events are reported to devd via system "PMU" and subsystem
"Battery", "AC" and "USB" such as connected/disconnected.
Some sensors values (power source voltage/current) are reported via
sysctl (dev.axp209_pmu.X.)
It also expose a gpioc node usable in kernel and userland. Only 3 of
the 4 GPIO are exposed (The GPIO3 is different and mostly unused on
boards). Most popular boards uses GPIO1 as a sense pin for OTG power.
Add a dtsi file that adds gpio-controller capability to the device as
upstream doesn't defined it and include it in our custom DTS.
Reviewed by: jmcneill
Approved by: cognet (mentor)
Differential Revision: https://reviews.freebsd.org/D6135
