mmc: meson: Assign the minimum clk rate as close to 400KHz as possible

The current code dealing with calculating mmc->f_min is a bit complicated.
Additionally, the attempt to set an initial clock rate should explicitly
use a rate between 100KHz to 400 KHz, according the (e)MMC/SD specs, which
it doesn't.

Fix the problem and clean up the code by using clk_round_rate() to pick the
nearest minimum rate to 400KHz (rounded down from 400kHz).

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
[Heiner: Changed from 100KHz to 400KHz to get a proper rounded rate]
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>