ppc: memory: Replace memory_region_init_ram with memory_region_allocate_system_memory

Commit 0b183fc8:"memory: move mem_path handling to
memory_region_allocate_system_memory" split memory_region_init_ram and
memory_region_init_ram_from_file. Also it moved mem-path handling a step
up from memory_region_init_ram to memory_region_allocate_system_memory.

Therefore for any board that uses memory_region_init_ram directly,
-mem-path is not supported.

Fix this by replacing memory_region_init_ram with
memory_region_allocate_system_memory.

Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>