powerpc/mm: MMU_FTR_NEED_DTLB_SW_LRU is only possible with CONFIG_PPC_83xx

Only mpc83xx will set MMU_FTR_NEED_DTLB_SW_LRU and its
definition is enclosed in #ifdef CONFIG_PPC_83xx.

Make MMU_FTR_NEED_DTLB_SW_LRU possible only when
CONFIG_PPC_83xx is set.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d01d7613664fafa43de1f1ae89924075bc24241c.1602489931.git.christophe.leroy@csgroup.eu