dt-bindings: mmc: rockchip-dw-mshc: add description for rk3568

Add "rockchip,rk3568-dw-mshc", "rockchip,rk3288-dw-mshc" compatibles for
mmc nodes on a rk3568 platform to rockchip-dw-mshc.yaml. Let's also take to
opportunity to clean up some old redundant comments around previous
compatibles.

Signed-off-by: Liang Chen <cl@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210429081151.17558-4-cl@rock-chips.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>