IB: add dependency on MMU

While building for NOMMU several link time issues were seen:

mm/built-in.o: In function `do_mmu_notifier_register':
usercopy.c:(.text+0x34d10): undefined reference to `mm_take_all_locks'
usercopy.c:(.text+0x34d9c): undefined reference to `mm_drop_all_locks'
usercopy.c:(.text+0x34de4): undefined reference to `mm_take_all_locks'
...
ERROR: "zap_vma_ptes" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!
ERROR: "zap_vma_ptes" [drivers/infiniband/hw/mlx4/mlx4_ib.ko] undefined!
ERROR: "can_do_mlock" [drivers/infiniband/core/ib_core.ko] undefined!
...
ERROR: "can_do_mlock" [drivers/infiniband/core/ib_core.ko] undefined!
...
ERROR: "zap_vma_ptes" [drivers/infiniband/hw/mlx4/mlx4_ib.ko] undefined!

It seems that Infiniband assumes that is it used in MMU configurations
only, so add dependency on CONFUG_MMU.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>