timer: hpet: make it support EHL/PSE CPU

Add config macro to set interrupt as level triggered for ARM CPUs
Merge all timer configures into one place, then no need to overwrite
hpet_timer_conf_get/set() functions in SoC layer
Make hpet_timer_comparator_set() as the only register access function
to implemented in the SoC layer

Signed-off-by: Dong Wang <dong.d.wang@intel.com>