boards: dts: Add fxos8700 interrupt bindings and fix sensor sample

Adds dts bindings for the fxos8700 interrupt pins to all boards that
have this sensor. The frdm_k64f and hexiwear_k64 connect both sensor
interrupt pins to the mcu, but the frdm_kw41z connects only one.

The fxos8700 driver is currently only aware of one sensor interrupt pin,
therefore the routing of INT1 or INT2 to the driver is handled in each
board's dts.fixup.

The fxos8700 sample application has been broken since the refactoring of
the mcux gpio driver to dts in commit
4e8f29f3. The sample is now fixed.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>