pci: Make PCIe Eject timeout configurable

PCIe mandates a 5 second delay between when the "Attention Button" is
pressed and when the associated device is detached; this is to allow
for the button to be pressed a second time to cancel the ejection. On
some systems this 5 second delay may not be desireable; so introduce a
hw.pci.pcie_hp_detach_timeout sysctl (which can also be set as a loader
tunable) which specifies the timeout in milliseconds (default 5000).
If set to zero, the device is detached immediately.

Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D49585

(cherry picked from commit 9be42ee6c9c741052bdc49f13bc953bf88a24423)