hal-common: require DerefMut for peripherals, not just Deref.

Otherwise you can create multiple drivers on the same singleton like this:

```rust
let mut input = Input::new(&pin, Pull::None);
let mut output = Output::new(&pin, Level::Low, Speed::Low);
input.is_high();
output.set_high();
input.is_high();
output.set_high();
```

Thanks @pennae for reporting.

embassy-hal-common/src/peripheral.rs

@@ -161,7 +161,7 @@ pub trait Peripheral: Sized {
     }
 }
 
-impl<'b, T: Deref> Peripheral for T
+impl<'b, T: DerefMut> Peripheral for T
 where
     T::Target: Peripheral,
 {