soc: xtensa: intel_adsp: ace: Fix build when CONFIG_MP_NUM_CPUS=1

When CONFIG_MP_MAX_NUM_CPUS=1, which it does for some tests, we will
get compiler warnings with soc_adsp_halt_cpu(), so only build it
when CONFIG_MP_MAX_NUM_CPUS > 1.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>