dts: st: stm32f030: add correct 'avgslope' property on die temp sensor

Add the missing 'avgslope' property to the DTSI for STM32F030/STM32F070.

This fixes improper results being returned by the driver: the correct
value for the average slope is 4.3mV/°C (4300 µV/°C), but the binding's
default value of 2.53mV/°C was used instead, since property was missing.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>