wm831x_power: Only register WM831x battery charger if enabled

Don't tell the system about the battery charger if it's not enabled, we
can reasonably assume that if the charger is not enabled then no battery
will ever be connected so reporting state is at best going to waste time
and at worst going to cause confusing information in the UI.

For simplicity and robustness we continue to register and handle interrupts
from the charger.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>