pci: Limit ASPM disable to ARM64

[kbowling]

Summary

ASPM can save double digit watts on NICs and other high end peripherals, it seems like a further ARM64 firmware investigation if these are getting disabled so limit the disable to that arch.

As a simple counterpoint, PHB4 on POWER9 supports ASPM and we'd want to support ASPM on any future PCIe implementations (RISC-V, etc) or know if there is some other issue at play.

[andrew]

We don't have any ASPM support so all this variable does is tell the ACPI code to lie to the firmware, and change what error code we return in the pci_disable_link_state fiunction in LinuxKPI.

[kbowling]

@andrew got it, thanks for clarifying