dt-bindings: mmc: rockchip-dw-mshc: add description for px30

Add "rockchip,px30-dw-mshc", "rockchip,rk3288-dw-mshc" for
dwmmc on px30 platform.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Liang Chen <cl@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>