arm: mvebu: helios4: mmc: move environment before 1M

Store the environment before 1M into the block device.
This constant is easier to remember, saves a little space,
and is in line with what SolidRun's 2018.01-based fork does for the
clearfog.

Signed-off-by: Josua Mayer <josua.mayer97@gmail.com>
[ Aditya Prayoga:
* Port forward]
Signed-off-by: Aditya Prayoga <aditya@kobol.io>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-By: Dennis Gilmore <dgilmore@redhat.com>
Reviewed-By: Dennis Gilmore <dgilmore@redhat.com>
Signed-off-by: Stefan Roese <sr@denx.de>