Documentation: rockchip-dw-mshc: add description for rk3228

Add "rockchip,rk3228-dw-mshc", "rockchip,rk3288-dw-mshc" for
dwmmc on rk322x platform.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>