drivers: sensors: mpu6050: add support for mpu9250

mpu9250 is a single package that contains both an mpu6050 6-axis
motion sensor and an ak8963 magnetometer. The two parts have
separate i2c addresses, yet despite the common mpu6050 component,
it has a different value in the "WHOAMI" register -- 0x71 instead
of 0x68.

This adds the additional chip id value in order to enable the use
of mpu9250.

Signed-off-by: Adam Serbinski <adam@serbinski.com>