thermal: tsens: Add SPDX license identifiers

The TSENS drivers use a GPL-2.0 license. Replace with equivalent SPDX
tags and delete the full license text.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>