IB/core: Introduce DECLARE_UVERBS_GLOBAL_METHODS

Introduce a new macro to be used for global methods on a singleton
object.

This macros sets internally the type_attrs to be NULL as such an object
can't be created.

Downstream patches from this series will use this macro.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>