pwm: update stm32 pwm to support LL clock control driver

After introducing STM32Cube based clock control driver for
stm32 family, update stm32 pwm driver to support it.
Once supported across the whole family, a clean up will be done.

Change-Id: I2ed38763b706e34ea1dceea50e97ce836dd9e195
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
2 files changed