Enable sdcard UHS modes, up to SDR50, on p20x based boards.
While the s905 supports SDR104 mode, it appears that the PCB of p20x
based boards can't cope with a rate as high as 200Mhz.
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>