iio: imu: Add support for the FXOS8700 IMU

FXOS8700CQ is a small, low-power, 3-axis linear accelerometer and 3-axis
magnetometer combined into a single package. The device features a
selectable I2C or point-to-point SPI serial interface with 14-bit
accelerometer and 16-bit magnetometer ADC resolution along with
smart-embedded functions.

FXOS8700CQ has dynamically selectable accelerationfull-scale ranges of
±2 g/±4 g/±8 g and a fixed magnetic measurement range of ±1200 μT.
Output data rates (ODR) from 1.563 Hz to 800 Hz are selectable by the user
for each sensor. Interleaved magnetic and acceleration data is available
at ODR rates of up to 400 Hz. FXOS8700CQ is available in a plastic QFN
package and it is guaranteed to operate over the extended temperature
range of –40 °C to +85 °C.

TODO: Trigger and IRQ configuration support

Datasheet:
  http://cache.freescale.com/files/sensors/doc/data_sheet/FXOS8700CQ.pdf

Signed-off-by: Robert Jones <rjones@gateworks.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>