overlays: Add i2c0 and i2c1 for regularity

The new i2c overlays for pi4 (i2c3, i2c4, i2c5, i2c6) have a
standardised interface that allows pin groups to be chosen
atomically rather than as individual pins. Add i2c0 and i2c1
overlays to fit the naming scheme and parameter usage, deprecating
i2c0-bcm2708 and i2c1-bcm2708.

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