power_supply: Add function to return system-wide power state

Certain drivers benefit from knowing whether the system is on ac or
battery, for instance when determining which backlight registers to
read. This adds a simple call to determine whether there's an online
power supply other than any batteries.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>