Add support for interrupts, sensors and GPIO for AXP209 PMIC.

Add support for interrupts, sensors and GPIO for AXP209 PMIC.
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