add missing ValidFunction implementation for DynFunction.

[ithinuel]

This missing impl effectively prevented anyone from using a Pin<_, DynFunction, _>.

[thejpster]

This now works:

let mut uart0_tx = pins
        .gpio0
        .reconfigure::<gpio::DynFunction, gpio::DynPullType>()
        .into_dyn_pin();

A helper method like into_really_dynamic_pin() or erase_all_the_type_state() would be useful :)