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>

include/zephyr/arch/xtensa/arch_inlines.h

@@ -50,9 +50,17 @@ static ALWAYS_INLINE uint32_t arch_proc_id(void)
 	return prid;
 }
 
+#ifdef CONFIG_SOC_HAS_RUNTIME_NUM_CPUS
+extern unsigned int soc_num_cpus;
+#endif
+
 static ALWAYS_INLINE unsigned int arch_num_cpus(void)
 {
+#ifdef CONFIG_SOC_HAS_RUNTIME_NUM_CPUS
+	return soc_num_cpus;
+#else
 	return CONFIG_MP_MAX_NUM_CPUS;
+#endif
 }
 
 #endif /* !_ASMLANGUAGE */

soc/Kconfig

@@ -53,3 +53,9 @@ config SOC_HAS_TIMING_FUNCTIONS
 	help
 	  Should be selected if SoC provides custom method for retrieving
 	  timestamps and cycle count.
+
+config SOC_HAS_RUNTIME_NUM_CPUS
+	bool
+	help
+	  Should be selected if SoC handles determining the number of CPUs
+	  at runtime.