ARM: dts: qcom: mdm9615*: add SPDX-License-Identifier

Replace the license blob by a clean SPDX-License-Identifier with GPL2+
or MIT even if X11 is specified in the original blob since the actual
license text corresponds to a MIT license.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220928-mdm9615-dt-schema-fixes-v4-2-dac2dfaac703@linaro.org