powerpc: use asm-generic/mmu_context.h for no-op implementations

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>