powerpc: Remove SYNC on non 6xx

SYNC is usefull for Powerpc 601 only. On everything else,
SYNC is empty.

Remove it from code that is not made to run on 6xx.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/27951fa6c9a8f80724d1bc81a6117ac32343a55d.1601362098.git.christophe.leroy@csgroup.eu