Battery module support for rk817

[0cc4m]

I have a device with a battery driver that is not currently supported by your battery module. A PineNote with a Rockchip RK817 battery controller. The driver is not upstreamed yet, but I think the changes are generic enough for this not to matter.

» ls /sys/class/power_supply/rk817-battery/
charge_empty_design  charge_full_design  charge_type                  constant_charge_voltage_max  current_boot  hwmon0  present  subsystem  uevent       voltage_boot        voltage_min_design  wakeup4
charge_full          charge_now          constant_charge_current_max  current_avg                  device        power   status   type       voltage_avg  voltage_max_design  voltage_ocv

It does not have a capacity, so it fails the initial check, and it does not have current_now or voltage_now.
I added charge_now to the initial file check, since capacity is only needed if charge_now doesn't exist.
I also added a check whether current_now or voltage_now exist, and if not attempt to read current_avg and voltage_avg

Please let me know what you think.

[Alexays]

Thx!