registers: Reexport ReadWriteable

The tock-registers::interfaces::ReadWriteable trait allows us to use
`modify()` function. It is useful for set flags in clean manner.

Signed-off-by: Akira Moroo <retrage01@gmail.com>

src/registers.rs

@@ -180,4 +180,4 @@ pub use vtcr_el2::VTCR_EL2;
 pub use vttbr_el2::VTTBR_EL2;
 
 #[doc(inline)]
-pub use tock_registers::interfaces::{Readable, Writeable};
+pub use tock_registers::interfaces::{ReadWriteable, Readable, Writeable};