arch: xtensa: Use Zephyr configuration options

Unconditionally use CONFIG_SIMULATOR_XTENSA to determine if XT_SIMULATOR
or XT_BOARD should be defined.

If CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC, also define XT_CLOCK_FREQ.  This
isn't ideal as the clock frequency might be changed in runtime and this
effectively makes it a constant.

Until we can control the clock frequency in runtime, this will suffice.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>