mlxsw: spectrum: Introduce basic set of flexible key blocks

Introduce basic set of Spectrum flexible key blocks. It contains blocks
needed to carry all elements defined so far.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>