soc: xtensa/intel_adsp: don't call soc_mp_init if MP_NUM_CPUS==1

There is no need to call soc_mp_init() if CONFIG_MP_NUM_CPUS
indicates only 1 CPU is being used. This also fixes an undefined
reference to soc_mp_init() since mp_cavs.c is not compiled
unless the build is targeting more than 1 CPU.

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