pcireg.h: Add values for ACS PCIe Extended capability.

These values come from section 7.7.11 ("ACS Extended Capability") of the
PCI Express Base Specification Revision 6.0, dated 16 Dec 2021.

Sponsored by: Chelsio Communications
Reviewed by: kib@
Differential Revision: https://reviews.freebsd.org/D37270

(cherry picked from commit b2700160cc4f5ff7d64926ea55a214b5ac970cf5)