soc: Add ability for SOC to specify runtime CPU detection

Add Kconfig SOC_HAS_RUNTIME_NUM_CPUS symbol that an SoC can
set to specify that it supports determining the number of CPUs
at runtime.

On xtensa add support for SOC_HAS_RUNTIME_NUM_CPUS and expose
soc_num_cpus that the SoC code should set early in boot as the
means to expose the number of cpus.

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