dt-bindings: iio: timer: stm32: add support for STM32H7

STM32H7 has slightly different triggers than previous versions.
Introduce st,stm32h7-timer-trigger compatible to handle this new variant.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>