scsi: ufs: Use freq table with devfreq

devfreq requires that the client operates on actual frequencies, not
only 0 and UMAX_INT and as such UFS brok with the introduction of
f1d981ea ("PM / devfreq: Use the available min/max frequency").

This patch registers the frequencies of the first clock with devfreq and
use these to determine if we're trying to step up or down.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>