iio: st_sensors: get platform data from device tree

Currently the STMicroelectronics sensors only support one single
platform data item: configuration of the DRDY (data ready) pin
for a particular design. Augment the core to prioritize and take
this information from the device tree if the parent device has an
assigned device node, else fall back to passed in platform data
(usually the default data).

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Denis CIOCCA <denis.ciocca@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>