drivers: stm32: clean up after stm23cube based clock control

After activation of cube based driver support on L4 and F3 series,
this commits performs the clean up of F3 and L4 relative code to
native clock control drivers.
Indirectly, it makes pwm driver supported de facto on F3 series

Change-Id: Idac17103a9b5ef6eab540719343cc8f5865f15fa
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>