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

Wire asm-generic/mmu_context.h to provide generic empty hooks for arch
code simplification.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>