mmc: sdhci: make max-frequency property in device tree work

Device tree provides option to specify the max freqency with property
"max-frequency" in dts and common parse function mmc_of_parse() will
parse it and use this value to set host->f_max to tell the MMC core
the maxinum frequency the host works.

However, current sdhci driver will finally overwrite this value with
host->max_clk regardless of the max-frequency property.

This patch makes sure not overwrite the max-frequency set from device
tree and do basic sanity check.

Signed-off-by: Dong Aisheng <aisheng.dong@freescale.com>
Reviewed-by: Johan Derycke <johan.derycke@barco.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>