dt-bindings: rockchip-dw-mshc: add description for rk3399

Add "rockchip,rk3399-dw-mshc", "rockchip,rk3288-dw-mshc" for
dwmmc on rk3399 platform.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>