Documentation: rockchip-dw-mshc: add description for rk3328

Add "rockchip,rk3328-dw-mshc", "rockchip,rk3288-dw-mshc" for
dwmmc on rk3328 platform.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>