m68k: 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: Dirk Mueller <dmueller@suse.com>
Message-Id: <CAL5wTH6X-GsT1AA8kEtP_e7oZWGZgi=fCcDfSs3wLgJN30DbUw@mail.gmail.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>