mmc: sdhci-of-dwcmshc: rename rk3568 to rk35xx

Prepare driver for rk3588 support by renaming the internal data
structures.

Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20220504213251.264819-11-sebastian.reichel@collabora.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>