ARM64: dts: meson-gxbb: p20x: enable sdcard UHS modes

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>