Fix soundness issue introduced in a previous soundness fix https://github.com/embassy-rs/embassy/pull/2602 .
PeripheralRef must not implement DerefMut itself, but the blanket impl must still require DerefMut. Otherwise
you can create two instances of a driver on the same uart by using `&my_uart`.
if you have `PeripheralRef<'a, AnyPIn>` for pin A, and `AnyPin` (owned) for pin B, you can `mem::swap` them.
so, getting access forever to pin A, just by "sacrificing" pin B
this defeats the point of PeripheralRef, which is if you got a `PeripheralRef<'a, T>` then you're only allowed to use the peripheral for `'a`.
Also some drivers rely on the fact only one instance of a singleton exists for soundness, so this is a soundness fix for those.
