rust: hpet: decode HPET registers into enums

Generalize timer_and_addr() to decode all registers into a single enum
HPETRegister, and use the TryInto derive to separate valid and
invalid values.

The main advantage lies in checking that all registers are enumerated
in the "match" statements.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/char/pl011/src/lib.rs

@@ -12,8 +12,6 @@
 //! See [`PL011State`](crate::device::PL011State) for the device model type and
 //! the [`registers`] module for register types.
 
-#![allow(clippy::upper_case_acronyms)]
-
 use qemu_api::c_str;
 
 mod device;