intel_th: SPDX-ify the documentation

Add the SPDX header to the Intel TH documentation.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/trace/intel_th.rst

+.. SPDX-License-Identifier: GPL-2.0
+
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 Intel(R) Trace Hub (TH)
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