power: supply: cpcap-battery: Add a battery driver

On the CPCAP PMIC we can use the ADCs for monitoring the battery,
and there is also a coulomb counter. So let's add basic support for
the battery driver.

I did not add any capacity prediction as that should probably be
done in the user space. Or at least user space should tell the kernel
some battery statistics and then the kernel driver could display the
capacity based on that.

Cc: devicetree@vger.kernel.org
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Michael Scott <michael.scott@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>