Mellanox: Add new mlxsw_spectrum firmware xx.2008.2946

Add latest verified version of Mellanox Spectrum-family switch firmware,
for Spectrum (13.2008.2946), Spectrum-2 (29.2008.2946) and Spectrum-3
(30.2008.2946).

The following issues have been fixed:

- A Spectrum-2 issue where control traffic generated on the switch would not
  egress a front panel port.
- Minimum shaper would not guarantee the requested bandwidth if the configured
  rate is 100Gbps+.
- On Spectrum-1, when bulk-updating FDB, a multicast tunnel entry update would
  be corrupted, and a unicast entry would not change VID.
- When ECMP or LAG are in symmetric hash mode, a packet would be steered to the
  wrong port.
- Obtaining activity for MC routes would sometimes exhibit a latency of >1us
  on Spectrum-3.

The following new features are available:

- Elephant & mice flow detection
- Register PMTDB, for querying of possible split-port scenarios
- Router interface profiles, which relax the requirement that all router
  interfaces have the ...