spi: rockchip: add bindings for rk3399 spi

Add devicetree bindings for Rockchip rk3399 spi which found on
Rockchip rk3399 SoCs.

Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@kernel.org>