arm64: defconfig: enable rockchip graphics

Enable the graphics-related options needed by Rockchip boards.
This includes the pwm-backlight which will be needed by the internal
displays used on Gru Chrome-devices.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Mark yao <mark.yao@rock-chips.com>
Signed-off-by: Olof Johansson <olof@lixom.net>