PCI: Add device-specific reset for Qualcomm SDX62/SDX65 modems

Qualcomm SDX62/SDX65 5G modems (17cb:0308) lack working reset methods for
VFIO passthrough scenarios. These devices have no FLR capability, advertise
NoSoftRst+ (blocking PM reset), and have broken bus reset.

Standard reset methods were already disabled for these devices by commit
6a4f64c3a3ad ("PCI: Avoid SBR for Qualcomm WCN6855/WCN7850 WiFi,
SDX62/SDX65 modems") because they were not working (quirk_no_bus_reset).

VFIO attempts to reset devices on every reassignment. Without a proper
reset capability, these devices never successfully initialize even on first
VM assignment.

Add a device-specific reset method using BAR-space hardware reset registers
that exist in these devices.

SDX62/SDX65 modem devices use MHI SoC reset via BAR0 (sequence from MHI
driver: mhi_soc_reset(), mhi_pci_reset_prepare()):

  - Write reset request to offset 0xb0
  - Wait 2 seconds for reset completion

These are true hardware reset mechanisms (not power management or firmware
error recovery), providing proper device reset for VFIO scenarios, allowing
to enforce isolation between successive passthrough users.

Testing shows stable operation over 109 VM crash/reset cycles. Device-
specific reset is position #1 in the reset hierarchy, so these Qualcomm
devices will use hardware reset as their primary reset method.

Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Acked-by: Alex Williamson <alex@shazbot.org>
Link: https://patch.msgid.link/20261005105436.83962-2-jtornosm@redhat.com

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