ti_armv7_keystone2: env: Add NFS loading support for PMMC and MON

NFS loading support has been added to the default environment for
most boot components, as PMMC and MON loading were added later they
did not originally get the NFS commands added, add these now.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>