ACPI: battery: add sysfs serial number

egrep serial /proc/acpi/battery/BAT0/info
serial number:           32090

serial number can tell you from the imminent danger
of beeing set on fire.

Signed-off-by: maximilian attems <max@stro.at>
Acked-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>