board: kontron: sl28: reorder mmc devices

Since linux commit 2e6cde96873253fd9eb0f20afd8ffd18278cff75 ("arm64:
dts: ls1028a: make the eMMC and SD card controllers use fixed indices")
mmc0 is the eMMC and mmc1 is the SD card. Also swap it in u-boot to
avoid any confusion by the user and to be aligned with linux.

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