rust/hpet: Rename HPETRegister to DecodedRegister

rust/hpet: Rename HPETRegister to DecodedRegister

HPETRegister represents the layout of register spaces of HPET timer
block and timer N, and is used to decode register address into register
enumeration.

To avoid confusion with the subsequently introduced HPETRegisters (that
is used to maintain values of HPET registers), rename HPETRegister to
DecodedRegister.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20251113051937.4017675-9-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>