ARM: dts: am57xx: Disable voltage switching for SD card

If UHS speed modes are enabled, a compatible SD card switches down to 1.8V
during enumeration. If after this a software reboot/crash takes place and
on-chip ROM tries to enumerate the SD card, the difference in
IO voltages (host @ 3.3V and card @ 1.8V) may end up damaging the card.

The fix for this is to have support for power cycling the card in
hardware (with a PORz/soft-reset line causing a power cycle of the card).
Because the beaglebone X15 (rev A,B and C), am57xx-evms and am57xx-idks don't
have this capability, disable voltage switching for these boards.

The major effect of this is that the maximum supported speed mode is now
high speed(50 MHz) down from SDR104(200 MHz).

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>