arm64: dts: meson: s400: fix emmc maximum rate

Limiting the HS200 rate on the s400 was just a way to mask that the
tuning setting were not correct. This seems to have been fixed with
the recent MMC driver update. We can now use HS200 at full speed.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>