CHROMIUM: power: dts: Add device tree binding for battery-system

CHROMIUM: power: dts: Add device tree binding for battery-system

This node brings together the various chargers and batteries into a single
unit called the battery system. It provides a unified view of the charge
state to user space.

This initial binding provides for the existing parameters in the driver.

BUG=chrome-os-partner:10617
TEST=build and boot to kernel prompt on snow

Change-Id: Ib5714d10dc2849779abb3f413deb5446fdfbdf81
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/40167
Reviewed-by: Benson Leung <bleung@chromium.org>