powerpc: Only use initrd_end as the limit for alloc_bottom if it's inside the RMO.

As the kernels and initrd's get bigger boot-loaders and possibly
kexec-tools will need to place the initrd outside the RMO.  When this
happens we end up with no lowmem and the boot doesn't get very far.

Only use initrd_end as the limit for alloc_bottom if it's inside the
RMO.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>