ARM: dts: qcom: msm8974: Add missing license headers

All other msm8974 dts files are licensed as GPL-2.0 so add the same
header to the files where it's missing.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220421211936.345168-3-luca@z3ntu.xyz