overlays: i2c-gpio: Fix the "bus" parameter

The "bus" parameter has two functions - providing unique names for
multiple instances of the overlay, and allowing the number of the bus
(i.e. "i2c-<bus>") to be specified. The second function hasn't worked
as intended because the overlay doesn't include a "reg" property and
the firmware intentionally won't create a "reg" property if one doesn't
already exist.

Allow the bus numbering scheme to work as intended by providing a "reg"
with a default value that means "the next available one".

See: https://github.com/raspberrypi/linux/issues/3062



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