drivers: timer: use sys_read64 to read HPET counter on 64 bits cpu

For 32 bit processor to read the 64 bits hpet counter, the HPET spec
2.4.7 suggest to read HPET counter high and low then checking the
high bits to decide if it rollovers or not.

But this logic seems to cause problem for 64 bits processor under SMP,
there is a possible one tick earier under tickless mode. It is likely
to be the cache coherence issue, because a mfence instruction before
reading the timer works.

So we change to read the 64 bits counter by sys_read64 on 64bit
processor to prevent this issue.

Fixes #49611

Signed-off-by: Enjia Mai <enjia.mai@intel.com>