overlays: i2c-sensor: Add comment re fragment gap

From rpi-6.1.y the i2c-sensor overlay includes support for the BNO055,
which occupies fragments 30 & 31. The driver for this devices does not
exist in the 5.15 kernel, so the rpi-5.15.y version of the overlay does
not include it.

Forward-porting new sensors added to rpi-5.15.y is made easier if the
same fragment numbers are used, so add a comment in the rpi-5.15.y
source explaining the gap.

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