AR7: prevent race between clock initialization and devices registration

ar7_regiser_devices needs ar7_clocks_init to have been called first, however clock.o is currently linked later due to its order in the Makefile, therefore ar7_clocks_init always gets called later than ar7_register_devices because both have the same initcall level. Fix this by moving ar7_register_devices to the right initcall level.

---

index 1d4a466..566f2d7 100644
@@ -647,4 +647,4 @@ static int __init ar7_register_devices(void)
 
 	return 0;
 }
-arch_initcall(ar7_register_devices);
+device_initcall(ar7_register_devices);