IB/uverbs: Add macros to simplify adding driver specific attributes

Previously, adding driver specific attributes required drivers to
declare all the hierarchy - object tree, object, methods and the
attributes themselves. A common use case is adding a few attributes to
an existing common method.
In order to simplify the driver's code, we add some macros to do all
these declarations automatically.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>