intel_th: Update ABI documentation

Commit a753bfcfdb1f3 ("intel_th: Make the switch allocate its subdevices")
changed the behavior so that the output port devices are created only for
the ports reported by the hardware and their initial state is "unassigned"
until a corresponding output port driver is loaded. Reflect this fact in
the ABI documentation.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reported-by: Ricardo Neri <ricardo.neri@intel.com>

intel_th: Add driver infrastructure for Intel(R) Trace Hub devices

Intel(R) Trace Hub (TH) is a set of hardware blocks (subdevices) that
produce, switch and output trace data from multiple hardware and
software sources over several types of trace output ports encoded
in System Trace Protocol (MIPI STPv2) and is intended to perform
full system debugging.

For these subdevices, we create a bus, where they can be discovered
and configured by userspace software.

This patch creates this bus infrastructure, three types of devices
(source, output, switch), resource allocation, some callback mechanisms
to facilitate communication between the subdevices' drivers and some
common sysfs attributes.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>