overlays: i2c-gpio: Support for multiple instances

Add a 'bus' parameter that, if set to a unique, non-zero
value, allows multiple i2c-gpio instances to coexist. The chosen
value doesn't determine the /dev/i2c-* value, but starting with
1 or 2 and counting upwards seems sensible. N.B. The bus parameter
has a default value of zero, so one instance doesn't need to specify
a value.

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