intel_th: Add "rtit" source device

In some versions of Intel TH, the Software Trace Hub (STH) has a second
MMIO BAR dedicated to the input from Intel PT. This calls for a new
subdevice that will be enumerated if the corresponding BAR is present.

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