thunderbolt: Add control channel interface

Thunderbolt devices are configured by reading/writing into their
configuration space (similar to pci). This is done by sending packets
through the NHI (native host interface) onto the control channel.

This patch handles the low level packet based protocol and exposes
higher level operations like tb_cfg_read/tb_cfg_write.

Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>