spapr_nvram: Pre-initialize the NVRAM to support the -prom-env parameter

In case we do not load the NVRAM contents from a file and the user
specified the "-prom-env" parameter, use the new CHRP NVRAM helper
functions to pre-initialize the NVRAM partitions, so that the SLOF
firmware now can pick up the environment variables from the -prom-env
parameter, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
