iio: st_sensors: add debugfs register read hook

This adds a debugfs hook to read/write registers in the ST
sensors using debugfs. Proved to be awesome help when trying
to debug why IRQs do not arrive.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>