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

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Cc: openrisc@lists.librecores.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>