The eMMC controller seem to have a maximum frequency of 200MHz, while the
regular MMC controllers are capped at 150MHz.

Since older SoCs cannot go that high, we cannot change the default maximum
frequency, but fortunately for us we have a property for that in the DT.

This also has the side effect of allowing to use the MMC HS200 and SD
SDR104 modes for the boards that support it (with either 1.2v or 1.8v IOs).
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Tested-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Acked-by: Chen-Yu Tsai <wens@csie.org>