arm64: dts: qcom: sc7180: Add SoC name to compatible

Running `make dtbs_check` yells because qcom.yaml says that we should
have:

- items:
    - enum:
        - qcom,sc7180-idp
    - const: qcom,sc7180

...but we're missing "qcom,sc7180".  Add it.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Fixes: 90db71e480 ("arm64: dts: sc7180: Add minimal dts/dtsi files for SC7180 soc")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191212113540.1.I158061c65974bf0f653ceb79b442b76a1fd64868@changeid
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

arch/arm64/boot/dts/qcom/sc7180-idp.dts

@@ -14,7 +14,7 @@
 
 / {
 	model = "Qualcomm Technologies, Inc. SC7180 IDP";
-	compatible = "qcom,sc7180-idp";
+	compatible = "qcom,sc7180-idp", "qcom,sc7180";
 
 	aliases {
 		hsuart0 = &uart3;