arm64: dts: rockchip: add a standalone version of the rk3399 sapphire

While the sapphire board is a system-on-module and mostly used with the
excavator baseboard, it is also possible to use it standalone without
any base. So add a board-variant for this type.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Vicente Bergas <vicencb@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>