Add a loader tunable (hw.pci.enable_pcie_hp) to disable PCI-e HotPlug.

Some systems and/or devices (such as riser cards) do not include a
non-compliant implementation of PCI-e HotPlug that can result in devices
not being attached (e.g. the HotPlug code might assume that a card is
being unplugged and will power the slot off and detach it). This
tunable can be set to 0 to disable support for PCI-e HotPlug ignoring
the incorrect HotPlug state on these slots.

PR: 211081
Reported by: Sergey Renkas <serg_ic@mail.ru> (SuperMicro X7 riser card)
Reported by: Jeffrey E Pieper <jeffrey.e.pieper@intel.com>

	 	(Intel X520 adapter)

MFC after: 1 week
Relnotes: yes