powerpc: sparse: static-ify some things

As sparse suggests, these should be made static.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>