mmc: sdhci-of-dwcmshc: properly determine max clock on Rockchip

Currently .get_max_clock returns the current clock rate for cclk_emmc
on rk35xx, thus max clock gets set to whatever bootloader set it to.

In case of u-boot, it is intentionally reset to 50 MHz if it boots
from eMMC, see mmc_deinit() in u-boot sources. As a result, HS200 and
HS400 modes are never selected by Linux, because dwcmshc_rk35xx_postinit
clears appropriate caps if host->mmc->f_max is < 52MHz

cclk_emmc is not a fixed clock on rk35xx, so using
sdhci_pltfm_clk_get_max_clock is not appropriate here.

Implement rk35xx_get_max_clock that returns actual max clock for cclk_emmc.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20230310010349.509132-1-anarsoul@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>