iio: Add bindings for STM32 timer trigger driver

Define bindings for STM32 timer trigger

version 8:
- reword "reg" parameter description

version 4:
- remove triggers enumeration from DT
- add reg parameter

version 3:
- change file name
- add cross reference with mfd bindings

version 2:
- only keep one compatible
- add DT parameters to set lists of the triggers:
  one list describe the triggers created by the device
  another one give the triggers accepted by the device

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>