[pfcwd] Use XGS PFC generator for NH-4210

[deerao02]

Description of PR

Summary:
Use the Broadcom XGS PFC generator for Nexthop NH-4210 SONiC fanouts by mapping the NH-4210-F-O256 HWSKU to Tomahawk6.

Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
  • Skipped for non-supported platforms
• Test case improvement

Back port request

• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605

Tracking issue/work item for backport/cherry-pick request (GitHub issue or Microsoft ADO): N/A
Failure type: day-one issue

Tested branch

• master
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• N/A

Test result

• Confirmed the generated fanout command uses pfc_gen_brcm_xgs.py -c Tomahawk6 instead of the generic PFC generator.
• On vms13-lt2-nh4210-1 (lt2-u32d128, str4-nh4210-01), the all-port PFC storm test detected storm on 142/160 ports, exceeding the 75% threshold.
• The IPv4 parameterization passed. The IPv6 parameterization also reached the storm threshold but later failed during watchdog restoration; that recovery failure is separate from generator selection.
• Python compilation, template-selection checks, and git diff --check passed.

Approach

What is the motivation for this PR?

The Nexthop NH-4210 fanout uses a Broadcom Tomahawk6 ASIC. Without an HWSKU-to-chip mapping, the PFC storm helper does not recognize this fanout as XGS-capable and selects the generic generator path. On the NH-4210 LT2 testbed, that path did not generate enough PFC traffic for the DUT to detect a storm.

How did you do it?

Added NH-4210-F-O256 to get_chip_name_if_asic_pfc_storm_supported() with the Tomahawk6 chip name. The existing SONiC XGS path then deploys pfc_gen_brcm_xgs.py and renders the XGS-compatible start and stop templates with the correct chip argument.

How did you verify/test it?

Validated the all-port PFC watchdog storm test on vms13-lt2-nh4210-1. The fanout ran pfc_gen_brcm_xgs.py -c Tomahawk6, and storm detection reached 142/160 ports. The IPv4 test passed; IPv6 reached storm state but failed later during restore. Also verified Python syntax and the LT2 start/stop template selection locally.

Any platform specific information?

Nexthop NH-4210 HWSKU: NH-4210-F-O256; Broadcom ASIC: Tomahawk6.

Supported testbed topology if it's a new test case?

N/A; this updates generator selection for existing PFC watchdog tests on LT2.

Documentation

N/A; no user-facing interface or new test case.

[mssonicbld]

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[bingwang-ms]

@gordon-nexthop FYI.