board: sl28: disable HS400 mode
Since commit 8ee802f899ef ("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>