powerpc/83xx: Also save/restore SPRG4-7 during suspend

The 83xx has 8 SPRG registers and uses at least SPRG4
for DTLB handling LRU.

Fixes: 2319f123

 ("powerpc/mm: e300c2/c3/c4 TLB errata workaround")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>