dt-bindings: mmc: sdhci-of-dwcmhsc: Add rk3588

Add compatible value for the Rockchip rk3588 dwcmshc controller.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220422170920.401914-8-sebastian.reichel@collabora.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>