mmc: sdhci: switch from programmable clock mode to divided one if needed

In programmable mode, if the clock frequency is too high, the divider
can be too small to meet the clock frequency requirement especially to
init the SD card. In this case, switch to the divided clock mode.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>