board: sl28: disable HS400 mode

Since commit 8ee802f8 ("mmc: fsl_esdhc: make sure delay chain locked
for HS400") HS400 mode is unreliable on LS1028A SoCs. Some workarounds are
missing for this SoC.

Disable HS400 mode for now.

Signed-off-by: Michael Walle <michael@walle.cc>