geam6ul: Add mmc_late_init

Let the runtime code can set the mmcdev and mmcroot based
on the devno using mmc_get_env_dev instead of defining
separately in build-time configs using mmc_late_init func.

Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>

include/configs/imx6ul_geam.h

@@ -44,9 +44,7 @@
 	"fdt_file=" CONFIG_DEFAULT_FDT_FILE "\0" \
 	"fdt_addr=0x87800000\0" \
 	"boot_fdt=try\0" \
-	"mmcdev=0\0" \
 	"mmcpart=1\0" \
-	"mmcroot=/dev/mmcblk0p2 rootwait rw\0" \
 	"nandroot=ubi0:rootfs rootfstype=ubifs\0" \
 	"mmcautodetect=yes\0" \
 	"mmcargs=setenv bootargs console=${console},${baudrate} " \