include/configs: ls1012a: add pfe stop command

This adds pfe stop command before boot linux kernel
to fix a problem that ethernet won't be workable
after enter linux. Because there is a h/w limitation
in LS1012A PFE, to re-initialize PFE it has to be
safe shutdown, these steps are done through PFE stop.

Signed-off-by: Biwen Li <biwen.li@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>