pci: Add helper routines to manage PME in device drivers

pci_has_pm is a quick check that returns true if a PCI device supports
the power management capability.

pci_enable_pme can be used in DEVICE_SUSPEND driver methods to enable
PME# during suspend.

Reviewed by: Krzysztof Galazka <krzysztof.galazka@intel.com>x
Differential Revision: https://reviews.freebsd.org/D49250

(cherry picked from commit e5cbf0e881fa1851912be77c62aa7ab17f844c3d)