So the M7 is tasked with accessing information from three sensors, and with Chipworks' help we found these too:

Bosch Sensortech BMA220 3-axis accelerometer

STMicroelectronics 3-axis gyroscope

AKM AK8963 3-axis magnetometer

After collecting information from the accelerometer, gyroscope, and magnetometer, the M7 performs some matrix math processing magic to produce an absolute orientation of the phone relative to the world. This data is then passed to the A7 in a neat package, probably in the form of three headings (roll, pitch, and yaw).

Using the A7 to monitor this sort of data would be mega-overkill, so the M7 was introduced to maintain a constant, low-power watch over these sensors.