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

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Matt Turner <mattst88@gmail.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: linux-alpha@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>