smp: limit the scope of some SMP-only functions

z_smp_init() is only available if CONFIG_SMP is defined,
smp_timer_init() also depends on two Kconfig parameters. Also make it
conditional in cavs_timer.c. Also clarify some SMP-related comments
there.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>