powerpc/32s: Move _tlbie() and _tlbia() prototypes to tlbflush.h

In order to use _tlbie() and _tlbia() directly
from asm/book3s/32/tlbflush.h, move their prototypes
from mm/mm_decl.h to there.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/867587af929973ad65f8ef6972f2474a80c1737a.1603348103.git.christophe.leroy@csgroup.eu