mlxsw: Introduce flex key elements for Spectrum-4

Spectrum-4 ASIC will support more virtual routers and local ports
compared to the existing ASICs. Therefore, the virtual router and local
port ACL key elements need to be increased.

Introduce new key elements for Spectrum-4 to be aligned with the elements
used already for other Spectrum ASICs.

The key blocks layout is the same for Spectrum-4, so use the existing
code for encode_block() and clear_block(), just create separate blocks.

Note that size of `VIRT_ROUTER_MSB` is 4 bits in Spectrum-4,
therefore declare it using `MLXSW_AFK_ELEMENT_INST_U32()`, in order to
be able to set `.avoid_size_check` to true.
Otherwise, `mlxsw_afk_blocks_check()` will fail and warn.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>