mmc: mmci: add f_max to variant structure

Some of the controller have maximum supported frequency, This patch adds
support in variant data structure to specify such restrictions. This
gives more flexibility in calculating the f_max before passing it to
mmc-core.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>