soc: xtensa: intel_adsp: Refine magic key initialization for ACE 1.5

Improve the initialization process for setting the magic key in ACE 1.5.

This update modifies the way the magic key is written to the global
variable:
- Utilizes a volatile pointer to ensure the key is directly written to
  the intended memory location.
- Ensures the cache is flushed after the key is set to guarantee
  consistency between the cache and main memory.

Signed-off-by: Tomasz Leman <tomasz.m.leman@intel.com>

soc/xtensa/intel_adsp/ace/multiprocessing.c

@@ -92,8 +92,10 @@ void soc_mp_init(void)
 	/* Set the core 0 active */
 	soc_cpus_active[0] = true;
 #if CONFIG_ACE_VERSION_1_5
-	g_key_read_holder = INTEL_ADSP_ACE15_MAGIC_KEY;
-	sys_cache_data_flush_range(&g_key_read_holder, sizeof(g_key_read_holder));
+	volatile uint32_t *key_write_ptr = z_soc_cached_ptr(&g_key_read_holder);
+
+	*key_write_ptr = INTEL_ADSP_ACE15_MAGIC_KEY;
+	sys_cache_data_flush_range(key_write_ptr, sizeof(g_key_read_holder));
 #endif /* CONFIG_ACE_VERSION_1_5 */
 }