mmc: sdhci: Let a driver override timeout clock frequency

Let a driver override the timeout clock frequency by
populating it before calling sdhci_add_host().  Note
the value will otherwise be zero because sdhci_host is
zeroed when allocated.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>