drivers: sensor: stm32_temp: read calibration data in dedicated function

Move the reading of calibration data from manufacturing flash to a
dedicated function. This makes introduction of new STM32 series easier.

Additionally, use the sys_read16 primitive to read data instead of
manipulating and dereferencing raw manufacturing flash pointers.

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