arm64: dts: qcom: sm8250: correct compatible for sm8250-mtp

Qualcomm boards should define two compatible strings: one for board,
anoter one for SoC family. sm8250-mtp.dts lists just the board
compatible, which makes it incompatible with qcom.yaml schema.

Reviewed-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Fixes: 60378f1a ("arm64: dts: qcom: sm8250: Add sm8250 dts file")
Link: https://lore.kernel.org/r/20200930112133.2091505-1-dmitry.baryshkov@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>