dt-bindings: pwm: Add STM32 bindings

Define bindings for pwm-stm32

version 9:
- change commit message header

version 8:
- reword st,breakinput description.

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

version 2:
- use parameters instead of compatible of handle the hardware configuration

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>