IB/mlx5: Add support for GRE flow specification

This patch introduces support for the GRE flow spec and
allowing the creation of rules based on the protocol and
key fields that are part of GRE protocol header.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>