samples: bbc_microbit: line_follower_robot: Convert to use DEVICE_DT_GET

Move to use DEVICE_DT_GET instead of device_get_binding as
we work on phasing out use of DTS 'label' property.

The I2C & GPIO devices are not represnted in devicetree and thus we
aren't converting this sample over to i2c_dt_spec or gpio_dt_spec.

Signed-off-by: Kumar Gala <galak@kernel.org>