Reserve bits 26-31 for internal use by low-level drivers. Two such
bits are used in the mlx4_b driver SR-IOV implementation.

These enum additions guarantee that the core layer will never use
these bits, so that low level drivers may safely make use of them.
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>