dts: bindings: stm32c0-temp-cal: force new series to define 'avgslope'

Set the 'avgslope' property of 'st,stm32c0-temp-cal' 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 (stm32f030.dtsi / stm32c0.dtsi), and both
of these already set 'avgslope' explicitly.

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