mmc: core: Use freq table with devfreq

Register the min/max frequencies with devfreq and use
these to determine if we're trying to step up or down.

Change-Id: Id1f49ad4b0e64967e4a72fb2c86df0a6550e1bec
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>