arm64: dts: qcom: pm6350: Include header for KEY_POWER

Make pm6350.dtsi self-contained by including input.h, needed for the
KEY_POWER constant used to define the power key.

Fixes: d8a3c775 ("arm64: dts: qcom: Add PM6350 PMIC")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Reviewed-by: Luca Weiss <luca.weiss@fairphone.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221030073232.22726-5-marijn.suijten@somainline.org