powerpc: Fix device tree claim code

I have a box that fails in OF during boot with:

DEFAULT CATCH!, exception-handler=fff00400
at   %SRR0: 49424d2c4c6f6768   %SRR1: 800000004000b002

ie "IBM,Logh". OF got corrupted with a device tree string.

Looking at make_room and alloc_up, we claim the first chunk (1 MB)
but we never claim any more. mem_end is always set to alloc_top
which is the top of our available address space, guaranteeing we will
never call alloc_up and claim more memory.

Also alloc_up wasn't setting alloc_bottom to the bottom of the
available address space.

This doesn't help the box to boot, but we at least fail with
an obvious error. We could relocate the device tree in a future
patch.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: <stable@kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>