amd-xgbe: add PCI power management for S0i3 support

amd-xgbe: add PCI power management for S0i3 support

The current suspend/resume implementation does not correctly handle PCI
device power state transitions, which prevents AMD platforms from
reaching the deepest suspend state (S0i3) when the amd-xgbe driver is
enabled.

In particular, the amd_pmc driver reports:

  "Last suspend didn't reach deepest state"

when this device is present.

Implement proper PCI power management operations following the standard
PCI PM model so that the device can be cleanly powered down and resumed.

Suspend path:
- Power down the network interface
- Put the PHY into low-power mode
- Disable bus mastering to prevent DMA activity
- Save PCI configuration space
- Disable the PCI device
- Disable wake from D3 (S0i3 does not require Wake-on-LAN)
- Set the device to D3hot

Resume path:
- Restore the PCI power state to D0
- Restore PCI configuration space
- Enable the PCI device
- Re-enable bus mastering
- Re-enable device interrupts
- Clear the PHY low-power mode
- Power up the network interface

This allows systems using amd-xgbe to reach the deepest suspend state
when entering modern standby (S0i3).

Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Link: https://patch.msgid.link/20260308092851.1510214-3-Raju.Rangoju@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>