net/mlx5: DR, Refactor VXLAN GPE flex parser tunnel code for SW steering

Refactor flex parser tunnel code:
 - Add definition for flex parser tunneling header for VXLAN-GPE
 - Use macros for VXLAN-GPE SW steering when building STE
 - Refactor the code to reflect that this is a VXLAN GPE
   only code and not a general flex parser code.
   This also significantly simplifies addition of more
   flex parser protocols, such as Geneve.

Signed-off-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Reviewed-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>