power_supply: add CHARGE_COUNTER property and olpc_battery support for it

This adds PROP_CHARGE_COUNTER to the power supply class (documenting it
as well).  The OLPC battery driver uses this for spitting out its ACR
values (in uAh).  We have some rounding errors (the data sheet claims
416.7, the math actually works out to 416.666667, so we're forced to
choose between overflows or precision loss.  I chose precision loss,
and stuck w/ data sheet values), but I don't think anyone will care
that much.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>