olpc_battery: Add support for CHARGE_NOW

CHARGE_NOW is needed by some user space software (read: UPower) for
internal calculations.

This patch violates the power supply class definition (as we already
do for CAPACITY though it isn't as obvious there), but this is the best
we can do without adding rather sophisticated algorithms to either the EC
or UPower.

Signed-off-by: Sascha Silbe <sascha-pgp@silbe.org>
Signed-off-by: Paul Fox <pgf@laptop.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>