mlxsw: core: Introduce flexible keys support

Hardware supports matching on so called "flexible keys". The idea is to
assemble an optimal key to use for matching according to the fields in
packet (elements) requested by user. Certain sets of elements are
combined into pre-defined blocks. There is a picker to find needed blocks.
Keys consist of 1..n blocks.

Alongside with that, an initial portion of elements is introduced in order
to be able to offload basic cls_flower rules.

Picked keys are cached so multiple rules could share them.

There is an encode function provided that takes care of encoding key and
mask values according to given key.

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