RDMA/core: Add optional access flags range

Define a range of access flags that are defined to be optional, both
uverbs and drivers should enable getting them and use if they are
applicable

This will be used, for example, for the relaxed ordering access flag which
unsupporting drivers can ignore.

Link: https://lore.kernel.org/r/1578506740-22188-7-git-send-email-yishaih@mellanox.com
Signed-off-by: Michael Guralnik <michaelgur@mellanox.com>
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>