bluetooth: Add CPF attribute to BAS battery level.

BAS v1.1, section 3.1.2.1 notes that a CPF should be added if the
device has more that one instance of the Battery Service, so
add one with the `main` description so that apps can add additional
Battery Level services with other descriptions, e.g. `auxiliary`.

Signed-off-by: Peter Johanson <peter@peterjohanson.com>