iio: trigger: stm32-timer: add enable attribute

In order to use encoder mode, timers needs to be enabled (e.g. CEN bit)
along with peripheral clock.
Add IIO_CHAN_INFO_ENABLE attribute to handle this.
Also, in triggered mode, CEN bit is set automatically in hardware.
Then clock must be enabled before starting triggered mode.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>