power_supply: Add a charge_type property, and use it for olpc driver

This adds a new sysfs file called 'charge_type' which displays the
type of charging (unknown, n/a, trickle charge, or fast charging).

This allows things like battery diagnostics to determine what the
battery/EC is doing without resorting to changing the 'status' sysfs
output.

Signed-off-by: Andres Salomon <dilinger@collabora.co.uk>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>