[POWERPC] Remove unused machine call outs

When we moved to arch/powerpc we actively tried to avoid using the
ppc_md.setup_io_mappings().  Currently no board ports use it so let's
remove it to avoid any new boards using it.

Also, remove early_serial_map() since we don't even have a call out for
it in arch/powerpc.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>