tock-registers: fix compiling with latest nightly.

At least since nightly (070e459c2 2021-05-11) the tock-registers crate fails
to compile stating that `const_fn` 'feature has been removed'.

This commit fixes this by specializing the feature to `const_fn_trait_bound`.

Author: Cryptjar

libraries/tock-register-interface/src/lib.rs

@@ -2,7 +2,7 @@
 //!
 //!
 
-#![feature(const_fn)]
+#![feature(const_fn_trait_bound)]
 #![no_std]
 
 pub mod macros;