tock-registers: make Field::mask field public

This partially reverts commit 89505ae ("Make the Field::mask
and FieldValue::mask fields private.") introduced through GitHub pull
request tock/tock#1939.

In order to be able to implement atomic RISC-V CSR bit-field-set and
bit-field-clear operations which operate on tock-register's Fields,
access to the Field::mask field is required.

Signed-off-by: Leon Schuermann <[email protected]>

Author: lschuermann

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

@@ -520,7 +520,7 @@ impl<T: IntLike, R: RegisterLongName> InMemoryRegister<T, R> {
 /// For the Field, the mask is unshifted, ie. the LSB should always be set.
 #[derive(Copy, Clone)]
 pub struct Field<T: IntLike, R: RegisterLongName> {
-    mask: T,
+    pub mask: T,
     pub shift: usize,
     associated_register: PhantomData<R>,
 }