rust/timer/hpet: add basic HPET timer and HPETState

rust/timer/hpet: add basic HPET timer and HPETState

Add the HPETTimer and HPETState (HPET timer block), along with their
basic methods and register definitions.

This is in preparation for supporting the QAPI interfaces.

Note, wrap all items in HPETState that may be changed in the callback
called by C code into the BqlCell/BqlRefCell.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250210030051.2562726-9-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>