board: sl28: enable HS400 mode again

Now that it is working reliable on the LS1028A SoC, reenable support for
it.

Signed-off-by: Michael Walle <michael@walle.cc>
diff --git a/configs/kontron_sl28_defconfig b/configs/kontron_sl28_defconfig
index 0c6c191..1c781e0 100644
--- a/configs/kontron_sl28_defconfig
+++ b/configs/kontron_sl28_defconfig
@@ -70,7 +70,7 @@
 CONFIG_I2C_DEFAULT_BUS_NUMBER=0
 CONFIG_I2C_MUX=y
 CONFIG_DM_MMC=y
-CONFIG_MMC_HS200_SUPPORT=y
+CONFIG_MMC_HS400_SUPPORT=y
 CONFIG_FSL_ESDHC=y
 CONFIG_FSL_ESDHC_SUPPORT_ADMA2=y
 CONFIG_DM_SPI_FLASH=y