powerpc/nvram: Scan partitions only once

nvram_scan_partitions() is called twice when initializing the "lnx,oops-log"
partition and the "ibm,rtas-log" partition. This fills the partition list
with duplicate entries. This patch moves the partition scan in the init
routine pseries_nvram_init_log_partitions() which is called only once.

Signed-off-by: Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>