hpet: accept 64-bit reads and writes

hpet: accept 64-bit reads and writes

Declare the MemoryRegionOps so that 64-bit reads and writes to the HPET
are received directly.  This makes it possible to unify the code to
process low and high parts: for 32-bit reads, extract the desired word;
for 32-bit writes, just merge the desired part into the old value and
proceed as with a 64-bit write.

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