net/mlx5: Map register values to restore objects

Currently reg_c0 lower 16 bits and reg_b are used to store the chain
id that missed in FDB and NIC tables accordingly. However, the
registers' values may index a restore object, rather than a single u32
value. Different object types can be used to restore mutually exclusive
contexts such as chain id and sample group id.

Use the mapping object to associate an index with a restore object
as a prestep for supporting additional restore types.

Signed-off-by: Chris Mi <cmi@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>