powerpc/mm: Add support for early ioremap on non-hash 64-bit processors

This adds some code to do early ioremap's using page tables instead of
bolting entries in the hash table. This will be used by the upcoming
64-bits BookE port.

The patch also changes the test for early vs. late ioremap to use
slab_is_available() instead of our old hackish mem_init_done.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>