pwm/stm32: add PWM driver for STM32 F1/F4/L4

Provide PWM driver for STM32 series F1/F4/L4.
Driver is ported on STM32Cube HAL and should support other STM32
series with minor updates.
Configuration is done so that PWM sample driver could be run on
all nucleo boards supporting PWM driver.

Change-Id: I6522a565451085df932e0eefd8404268380b5bfe
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>