xtensa: remove CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC from arch

CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC should be defined at the SoC
or the board level since Xtensa cores are high configurable.
The default is just for ISS (Instruction Set Simulator). So
remove it from the arch level.

The xt-sim board is the only one in tree that is targeting
the ISS, so add it there.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>