asm-generic: add generic MMU versions of mmu context functions

Many of these are no-ops on many architectures, so extend mmu_context.h
to cover MMU and NOMMU, and split the NOMMU bits out to nommu_context.h

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arch@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>