overlays: i2c-sensor: Add mpu6050 and mpu9250

Add support for the MPU6050 and MPU9250 IMU sensors to the i2c-sensor
container overlay. At the same time, deprecate the standalone mpu6050
overlay, but keep it around for now.

(Also fix a few typos spotted while editing)

See: https://github.com/raspberrypi/linux/issues/5325



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