powerpc/ps3: Add highmem region memory early

Real mode memory can be limited and runs out quickly as memory is allocated
during kernel startup.  Having the highmem available sooner fixes this.

This change simplifies the memory management code by converting from hotplug
memory to logical memory blocks.

Signed-off-by: Hector Martin <hector@marcansoft.com>
Signed-off-by: Andre Heider <a.heider@gmail.com>
Signed-off-by: Geoff Levand <geoff@infradead.org>