drivers: clock_control: stm32 fixed LSE system clock enable

On some stm32 mcus, the LSE is enabled as system clock (LSESYS)
only when the LSEON and LSERDY are both set.
The bit LSESYSEN is set in the RCC BDCR register
and the driver is waiting for the LSESYSRDY to be set.

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