ACPI: battery: add power_{now,avg} properties to power_class

ACPI has smart batteries, which work in units of energy and measure
rate of (dis)charge as power, thus it is not appropriate to export it
as a current_now. Current_now will still be exported to allow
for userland applications to match.

Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>