drivers: clock_control: some stm32 have a HSI48 fixed clock

For the stm32 devices that have a HSI48 clock,
the driver enables it, like any other fixed clock,
if needed and supported by the serie.
For stm32L0, SYSCFG VREFINT is also required.

Signed-off-by: Francois Ramu <francois.ramu@st.com>