mmc: sdhci-of-dwcmshc: add rockchip platform support

sdhci based synopsys MMC IP is also used on some rockchip platforms,
so add a basic support here.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Link: https://lore.kernel.org/r/1615879102-45919-3-git-send-email-shawn.lin@rock-chips.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>