power: supply: Add support for RN5T618/RC5T619 charger and fuel gauge

Both chips have charger and a fuel gauge.

This adds basic support for displaying the state of the battery and the
input power, settings are not modified. There are some defaults set via
OTP.

Charging also starts after plugging USB.

Known issues of the fuel gauge: There are drivers in the wild which disable
the fuel gauge at shutdown. If a kernel is booted without fuel gauge
support, after such a driver has been used, the fuel gauge will stay off
and decalibrate.
If this driver is used after that, it might display wrong values for charge
level.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>