drivers: sensor: mpu6050: scale settings to DT, adds sample rate setting

Moves the MPU6050 accel/gyro scale settings from KConfig to Devicetree.
Adds a new setting for the MPU6050 sample rate divider register and
transmits it to the sensor upon initialization.
This helps to reduce the interrupt firing rate when combined with the
data ready trigger.
A default division factor is provided which ensures compatibility with
existing applications.
The MPU6050 sample application is extended and used for hardware tests.

Signed-off-by: Tilmann Unte <unte@es-augsburg.de>