add missing ValidFunction implementation for DynFunction.

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

rp2040-hal/src/gpio/func.rs

@@ -2,7 +2,7 @@ use core::marker::PhantomData;
 
 use paste::paste;
 
-use super::pin::DynBankId;
+use super::{pin::DynBankId, DynPinId};
 
 pub(crate) mod func_sealed {
     use super::DynFunction;
@@ -173,6 +173,7 @@ impl DynFunction {
         }
     }
 }
+impl<P: super::pin::PinId> ValidFunction<DynPinId> for P {}
 macro_rules! pin_valid_func {
     ($bank:ident as $prefix:ident, [$head:ident $(, $func:ident)*], [$($name:tt),+]) => {
         pin_valid_func!($bank as $prefix, [$($func),*], [$($name),+]);