arch: rename arch_start_cpu() to arch_cpu_start()

Rename arch_start_cpu() to arch_cpu_start() so it belongs to
the "cpu" namespace.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
diff --git a/soc/espressif/esp32/esp32-mp.c b/soc/espressif/esp32/esp32-mp.c
index 7922e6f..c5ec588 100644
--- a/soc/espressif/esp32/esp32-mp.c
+++ b/soc/espressif/esp32/esp32-mp.c
@@ -243,7 +243,7 @@
 	}
 }
 
-void arch_start_cpu(int cpu_num, k_thread_stack_t *stack, int sz,
+void arch_cpu_start(int cpu_num, k_thread_stack_t *stack, int sz,
 		    arch_cpustart_t fn, void *arg)
 {
 	volatile struct cpustart_rec sr;