dt-bindings: mmc: rockchip-dw-mshc: add description for rk3308

dt-bindings: mmc: rockchip-dw-mshc: add description for rk3308

The description below is already in use for rk3308.dtsi,
but was somehow never added to a document, so add
"rockchip,rk3308-dw-mshc", "rockchip,rk3288-dw-mshc"
for mmc nodes on a rk3308 platform to rockchip-dw-mshc.yaml.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200116152230.29831-3-jbx6244@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>