AP_HAL: Add SPIDevice_MPU9250 as a device.
AP_HAL_Linux: Add support for SPIDevice_MPU9250

The Invensense MPU9250 SPI device is connected to
SPI1, thereby SPIDriver class should return the
corresponding device.
AP_InertialSensor_MPU9250: SPI userspace sensor driver.

InvenSense MPU9250 sensor driver and test code (available under
libraries/examples/).