Add the obvious compatible name to the existing I2C binding, and pair
it with the existing A31 fallback compatible string, as the devices
are compatible.

On the way use enums to group all compatible devices together.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>