MPU9250 9DOF sensor in SPI mode support

Currently this is used by my Draco target only, but maybe useful for any other board using this sensor in the future.
Placing of * is inconsistent within the whole file. Please rework according to kernel style guidelines:
>When declaring pointer data or a function that returns a pointer type, the
>preferred use of '*' is adjacent to the data name or function name and not
>adjacent to the type name."