The Snowball gyro and magnetometer DRDY (data ready) lines are used
as interrupt triggers by the IIO subsystem, triggering new data
retrieveal on edges on this signal. Thus assign them as the primary
interrupt in the device tree. The "other interrupt" coming out of
the sensors is for alarms and similar complex events, the DRDY is
for periodic measures, which is what we want.

After this the generic_buffer tool from IIO tools gives a nice
stream of readings after enabling the desired channels in sysfs.

Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>