rockchip: ram: update license for sdram driver

Rockchip may use this sdram copy of source code for both open source
and internal project, update the license to use both GPL2.0+ and
BSD-3 Clause.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/drivers/ram/rockchip/sdram_rk3188.c b/drivers/ram/rockchip/sdram_rk3188.c
index 3774abf..fdd500a 100644
--- a/drivers/ram/rockchip/sdram_rk3188.c
+++ b/drivers/ram/rockchip/sdram_rk3188.c
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause
 /*
  * (C) Copyright 2015 Google, Inc
  * Copyright 2014 Rockchip Inc.