ls2080: Exit dpaa only right before exiting U-Boot

On ls2080 we have a separate network fabric component which we need to
shut down before we enter Linux (or any other OS). Along with that also
comes configuration of the fabric using a description file.

Today we always stop and configure the fabric in the boot script and
(again) exit it on device tree generation. This works ok for the normal
booti case, but with bootefi the payload we're running may still want to
access the network.

So let's add a new fsl_mc command that defers configuration and stopping
the hardware to when we actually exit U-Boot, so that we can still use
the fabric from an EFI payload.

For existing boot scripts, nothing should change with this patch.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: York Sun <york.sun@nxp.com>
[agraf: Fix x86 build]