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

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Guo Ren <guoren@kernel.org>
Cc: linux-csky@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>