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

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: linux-snps-arc@lists.infradead.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>