power: supply: ds2760_battery: merge ds2760 supply driver with its w1 slave companion

This patch removes the w1 slave driver that used to register the w1 family
and instanciate a platform device at runtime. The code now lives in the
supply driver instead to avoid that level of indirection.

The old device name "ds2760-battery.0" is preserved, so userspace
applications can access the same virtual device nodes as before.

Note that because the w1 core does not currently have a framework for
suspend/resume, the driver now registers a PM notifier callback.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>