dts: am57xx-beagle-x15: disable UHS and HS200 support

The UHS modes are not supported in beagle-x15 because it's not possible to
switch the IO lines supply voltage to 1.8v.
Also HS200 cannot be supported on mmc2, because the IO lines of mmc2 are
connected to 3.3v.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>