pwm: Add driver for STM32 plaftorm

This driver adds support for PWM driver on STM32 platform.
The SoC have multiple instances of the hardware IP and each
of them could have small differences: number of channels,
complementary output, auto reload register size...

version 9:
- fix commit message header
- remove one space MODULE_ALIAS

version 8:
- fix comments done by Thierry on version 7

version 6:
- change st,breakinput parameter to make it usuable for stm32f7 too.

version 4:
- detect at probe time hardware capabilities
- fix comments done on v2 and v3
- use PWM atomic ops

version 2:
- only keep one comptatible
- use DT parameters to discover hardware block configuration

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>