dts: bindings: stm32-temp: force new series to define 'avgslope'

Set the 'avgslope' property of 'st,stm32-temp' to required, and remove its
default value, to ensure new series cannot be introduced without setting
the property to the correct value explicitly.

This change does not require any DTSI modification, because there are only
two files using this compatible (stm32f1.dtsi / stm32f2.dtsi), and both of
these already set 'avgslope' explicitly.

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