mmc: dw_mmc: Support setting f_min from host drivers

Host drivers may not be able to support frequencies as low as dw-mmc
supports. Unfortunately f_min isn't available when the drv_data->init
function is called, as the mmc_host struct hasn't been set up yet.

Support the host drivers saving the requested minimum frequency, so we
can later set f_min when it is available.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Link: https://lore.kernel.org/r/20220305215835.2210388-2-pgwipeout@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>