RDMA/core: Allow existing drivers to set one sysfs group per device

Currently many rdma drivers are creating device attribute files using
device_create_file() with device specific attributes.  Device specific
attributes should be exposed via well defined netlink device attributes in
future.

Introduce an API rdma_set_device_sysfs_group() for existing drivers to set
a group for sysfs attributes for legacy.

This API is only for exposing legacy attributes which existed for sometime
now.  New drivers should not be using this API and rather follow netlink
path.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>