Update fw-manager to support firmware upgrades for the new ASIC

[ganglyu]

Why I did it

The existing fw-manager does not support firmware upgrades for SPC6 ASICs.

Work item tracking

• Microsoft ADO (number only):

How I did it

1. Extended SpectrumFirmwareManager.run_firmware_update() to load the Spectrum kernel driver via sx-kernel.sh start before invoking mlxfwmanager, and unload it via sx-kernel.sh stop in a finally block after the burn completes. Loading the driver enables DMA-based flash access, which reduces burn time. Unloading after burn lets sx-kernel.service bring the driver up cleanly for syncd.
2. Changed the mlxfwmanager/flint device argument from self.pci_id (PCI BDF, e.g. 01:00.0) to the fixed MST path /dev/mst/mt53124_pciconf0 (PCI config access). This is required for SPC6 firmware burn per MFT/product specification.
3. Increased TimeoutSec in mlnx-fw-manager.service from 300 s to 900 s to accommodate SPC6 burn time, which can exceed 10 minutes.
4. Increased timeout_per_asic in firmware_coordinator.py from 600 s to 900 s to align with the service-level timeout.
5. Updated unit tests to mock _run_sx_kernel and added test_run_firmware_update_sx_kernel_start_fails_still_burns to verify graceful degradation when the driver load fails.

How to verify it

1. Run unit tests:
2. On SPC6 device, trigger firmware upgrade and confirm:

• Service completes without timeout
• Logs show sx-kernel.sh start before burn and sx-kernel.sh stop after
• syncd comes up cleanly after reboot

Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[mssonicbld]

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[mssonicbld]

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