AP_InertialSensor_MPU9150: Clock-based wait_for_sample() impl.

Adapt the I2C driver to a system clock based wait_for_sample()
implementation.

The sample rate of the sensor has been corrected to 800 Hz (could be
further pushed up to 1KHz).

Filters for the mag have also been created but remain commented until
the code for the mag is ready.
Fixes to make the ERLE layer work
AP_HAL_Linux: Correct SPIDriver devs
I2CDriver: include readRegistersMultiple.

AP_HAL_Linux implemments this new method.
AP_InertialSensor_L3G4200D: Clock-based wait_for_sample() impl.
AP_Motors_test: Adapt to test bench available