drivers: Bluetooth: Generalize IC-specific setup hook

In order to generalize the currently specialized nRF51 IC setup hook,
make the following changes:

- Generalize the hook to bt_ic_setup()
- Use a weak NOP version by default
- Move the currently existing one to the board folder

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>