fatal_types: define start of arch specific faults

Define the index where arch specific faults start. This is a specific
number to avoid arch specific fault values changing each time a new
generic fault is added.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>

include/zephyr/fatal_types.h

@@ -37,9 +37,8 @@ enum k_fatal_error_reason {
 	/** High severity software error */
 	K_ERR_KERNEL_PANIC,
 
-	/* TODO: add more codes for exception types that are common across
-	 * architectures
-	 */
+	/** Arch specific fatal errors */
+	K_ERR_ARCH_START = 16
 };
 
 /** @} */