intel_th: Document debug host mode

# and now you can collect the trace from the device node:

$ cat /dev/intel_th0/msc0 > my_stp_trace

Host Debugger Mode

==================

It is possible to configure the Trace Hub and control its trace

capture from a remote debug host, which should be connected via one of

the hardware debugging interfaces, which will then be used to both

control Intel Trace Hub and transfer its trace data to the debug host.

The driver needs to be told that such an arrangement is taking place

so that it does not touch any capture/port configuration and avoids

conflicting with the debug host's configuration accesses. The only

activity that the driver will perform in this mode is collecting

software traces to the Software Trace Hub (an stm class device). The

user is still responsible for setting up adequate master/channel

mappings that the decoder on the receiving end would recognize.

In order to enable the host mode, set the 'host_mode' parameter of the

'intel_th' kernel module to 'y'. None of the virtual output devices

will show up on the intel_th bus. Also, trace configuration and

capture controlling attribute groups of the 'gth' device will not be

exposed. The 'sth' device will operate as usual.