IB/mlx5: Initialize the parsing tree root without the help of uverbs

In order to have a custom parsing tree, a provider driver needs to
assign its parsing tree to ib_device specs_tree field. Otherwise, the
uverbs client assigns a common default parsing tree for it.
In downstream patches, the mlx5_ib driver gains a custom parsing tree,
which contains both the common objects and a new flags field for the
UVERBS_FLOW_ACTION_ESP_CREATE command.
This patch makes mlx5_ib assign its own tree to specs_root, which
later on will be extended.

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