overlays: Make the i2c-gpio overlay safe again

Like many overlays, the i2c-gpio overlay goes to efforts to avoid
generating warnings about #address-cells and #size-cells not
being defined, which it does by defining them. Unfortunately this
is fatal if they don't match what the system requires, and the
recent switch to #size-cells = 2 on 2711 made i2c-gpio very
dangerous.

In the absence of the knowledge of a clean way to fix this, just delete
the declarations and suffer the warnings.

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