overlays: mpu6050: Add 'addr' parameter

The mpu6050 starts up at address 0x68 by default, but can be set to
0x69 if the ADO pin is pulled high. Give the overlay an addr parameter
to allow devices at the alternate address to be used.

See: https://github.com/Hexxeh/rpi-firmware/issues/252



Signed-off-by: Phil Elwell <phil@raspberrypi.com>