rockchip: ram: update license for sdram driver

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>