Make ConfigurablePpi a subtrait of Ppi

[timokroeger]

This allows to create nice generic driver APIs with impl ConfigurablePpi arguments
Example:

fn link_pins(
    gpiote: &hal::gpiote::Gpiote,
    mut ppi: impl ConfigurablePpi,
    out_pin: Pin<Output<PushPull>>,
    in_pin: Pin<Input<PullUp>>,
) {
    // ...
}