powerpc/ps3: Add ps3_mm_set_repository_highmem

Add the new routine ps3_mm_set_repository_highmem() that saves highmem info to
the LV1 hypervisor registry so that the info will be available to second stage
OS's loaded by petitboot/kexec. FreeBSD and some Linux derivatives use
this feature.

Also, move the existing ps3_mm_get_repository_highmem() routine up in
the source file.

This implementation of ps3_mm_set_repository_highmem() assumes the repository
will have a single highmem region entry (at index 0).

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>