Provide newbus infrastructure for initiating device reset.

The methods BUS_RESET_PREPARE(), BUS_RESET(), and BUS_RESET_POST()
should be implemented by bus which can provide reset to a device. The
methods are described in inline doxygen comments.

Code only provides BUS_RESET_PREPARE() and BUS_RESET_POST() helpers
instead of default implementation, because actual bus needs to handle
device state around reset, while helpers provide the other half of
typical prepare/post code.

Reviewed by: imp (previous version), jhb (previous version)
Sponsored by: Mellanox Technologies
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D19646