mmc: sdhci-esdhc: calculate sdclk divider from controller clock

The SDCLK is divided down from the host controller clock. Host
controller clock may be different from the maximum SDCLK, so
get it from the platform, instead of just using the max SDCLK.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>