soc: intel_s1000: use CAVS DSP wall clock timer for SMP

The DSP wall clock timer is a timer driven directly by
external oscillator and is external to the CPU core(s).
It provides a common and synchronized counter for all
CPU cores (which is useful for SMP), instead of
indepedently running local core timer (xtensa_timer).

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