ARM64: dts: meson: reduce odroid-c2 eMMC maximum rate

Different modules maybe installed by the user on the eMMC connector
of the odroid-c2. While the red modules are working without an issue,
it seems some black modules (apparently Samsung based) are having
issue at 200MHz

While the tuning algorithm introduced in v4.14 enables high speed modes
on every other tested designs, it seems a problem remains for this
particular combination of board and eMMC module.

Lowering the maximum frequency of the eMMC on this board until we can
figure out a better solution.

Fixes: d341ca88 ("mmc: meson-gx: rework tuning function")
Suggested-by: Ellie Reeves <ellierevves@gmail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Cc: stable@vger.kernel.org
Signed-off-by: Kevin Hilman <khilman@baylibre.com>