pci: Only claim to support PCI ASPM on x86

We claim to support Active State Power Management, but don't appear to
do anything different in the kernel when it's enabled other than tell
the firmware we do.

This breaks VMware Fusion on Apple Silicon when it's enabled as it
expects the kernel to enable the ports. As it is reported to be needed
on some x86 servers keep it enabled there, but disable on non-x86
architectures.

Reported by: kp, tuexen
Reviewed by: tuexen, mav, imp, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48303