Staging: Add staging/rdma directory and update MAINTAINERS

Create the rdma directory in the staging area for use as we deprecate
some older drivers and as we bring in some new drivers that are in
need of work.  Update the MAINTAINERS file so that updates to these
files go to linux-rdma@vger.kernel.org.  Expected lifespan of this
directory is three releases for any deprecated drivers moved here
and an unknown, but theoretically bounded amount of time for the new
drivers as a new core RDMA transfer library needs to be written and
the drivers modified to use it in order for them to move out of this
directory.

Signed-off-by: Doug Ledford <dledford@redhat.com>