dts: bindings: stm32-temp*: align 'avgslope' to datasheet format

Change the STM32 Temperature Sensor bindings to accept the average slope
value in string form instead of integer. With this change, it is possible
to use the raw decimal value found in each MCU's datasheet instead of
needing to scale it (differently depending on series!). This also allows
regrouping the property in a single file to reduce duplication.

Also update all DTSI files affected by this change and the dietemp driver
to accept the property's new format.

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