Add support for HS400 mode.

The driver still misses support for tuning, therefore
highspeed modes like HS400 might not work under all
circumstances yet.

Successfully tested on a Odroid C2 (S905 GXBB).
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>