ARM: dts: qcom: mdm9615: add missing reg in cpu@0 node

Fixes cpu@0: 'reg' is a required property from dtbs check.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220928-mdm9615-dt-schema-fixes-v4-3-dac2dfaac703@linaro.org