CHROMIUM: power: chromeos_ec: charger driver.

CHROMIUM: power: chromeos_ec: charger driver.

On spring, the charger (tps65090) is abstracted via EC interface.

This CL creates a driver for the charger using EC's MKBP interface
(EC_CMD_POWER_INFO) and provides power supply property
POWER_SUPPLY_PROP_ONLINE.

Signed-off-by: Todd Broch <tbroch@chromium.org>
BUG=chrome-os-partner:17927
TEST=code compiles

Change-Id: I03c444e82771871f47dbd12a7f4f282c2e95be3d
Reviewed-on: https://gerrit.chromium.org/gerrit/44681
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Commit-Queue: Todd Broch <tbroch@chromium.org>
Tested-by: Todd Broch <tbroch@chromium.org>