Chipworks did some investigatin' and found the M7 to be an NXP LPC18A1, part of the LPC1800 series of high-performing ARM Cortex-M3 based microcontrollers.

Don't believe them? Check out the NXP inscription in the top right of the die photo.

From Chipworks: "The M7 is a new direction for Apple: in an effort to reduce power consumption, the M7 chip is dedicated to collecting and processing accelerometer, gyroscope and compass data."