earlycon: mark earlycon code as __used iif the caller is built-in

Keep earlycon related symbols only when CONFIG_SERIAL_EARLYCON is
enabled and the driver is built-in.  This will be helpful to clean
up ifdefs surrounding earlycon code in serial drivers.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/serial_core.h

@@ -352,9 +352,15 @@ struct earlycon_id {
 extern const struct earlycon_id __earlycon_table[];
 extern const struct earlycon_id __earlycon_table_end[];
 
+#if defined(CONFIG_SERIAL_EARLYCON) && !defined(MODULE)
+#define EARLYCON_USED_OR_UNUSED	__used
+#else
+#define EARLYCON_USED_OR_UNUSED	__maybe_unused
+#endif
+
 #define OF_EARLYCON_DECLARE(_name, compat, fn)				\
 	static const struct earlycon_id __UNIQUE_ID(__earlycon_##_name)	\
-	     __used __section(__earlycon_table)				\
+	     EARLYCON_USED_OR_UNUSED __section(__earlycon_table)	\
 		= { .name = __stringify(_name),				\
 		    .compatible = compat,				\
 		    .setup = fn  }