iio: gyro: add core driver for fxas21002c

Add core support for the NXP fxas21002c Tri-axis gyroscope,
using the iio subsystem. It supports PM operations, axis reading,
temperature, scale factor of the axis, high pass and low pass
filtering, and sampling frequency selection.

It will have extras modules to support the communication over i2c and
spi.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>