mmc: dw_mmc: set the supported max/min frequency

Both f_max and f_min will be informed for core layer to request
valid clock rate. But current setting from 'host->bus_hz' may
not represent the max/min frequency properly. Even if host can
actually support high speed than bus_hz, core layer will not
request clock rate over bus_hz. Basically, f_max/f_min can be set
with the values according to spec. And then host will make its best
effort to meet the rate.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>