drivers: clock_control: stm32: Modify file naming pattern

Change file naming pattern from BoardNameX_ll_clock.c to
clock_BoardNameX.c
File containing LL functions will have the "_ll_" naming
scheme (such as clock_stm32_ll_common.c and .h)

Signed-off-by: Yaël Boutreux <yael.boutreux@st.com>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>