powerpc: Remove RELOC() macro

Now we relocate prom_init.c on 64bit we can finally remove the
nasty RELOC() macro.

Finally a patch that I can claim has a net positive effect on
the kernel. It doesn't happen very often.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>