drivers: clock_control: Add stm32u0 clock control support

Reuse the file clock_stm32g0.c for the STM32U0 and
rename it to clock_stm32g0_u0.c
because the G0 and U0 series share the same clock control.

Signed-off-by: IBEN EL HADJ MESSAOUD Marwa <marwa.ibenelhadjmessaoud-ext@st.com>