iio: stm32 trigger: Add support for TRGO2 triggers

Add support for TRGO2 trigger that can be found on STM32F7.
Add additional master modes supported by TRGO2.
Register additional "tim[1/8]_trgo2" triggers for timer1 & timer8.
Detect TRGO2 timer capability (master mode selection 2).

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>