dt-bindings: arm: sunxi: Combine board variants into enums

dt-bindings: arm: sunxi: Combine board variants into enums

The common style in all bindings for enumerating similar variants is to
use 'enum', not 'oneOf', so all boards of same variant or using same SoM
should be grouped under enum.  It is more readable, more compact and
makes easier to find actual devices from the same family/type.  Also
there is completely no point to repeat the compatible in the description
(incircuit,icnova-a20-adb4006 implies this is "ICnova A20 ADB4006" and
pine64,pinephone-1.2 implies "Pine64 PinePhone (1.2)").

Combine two easily visible cases: ICnova A20 and Pine64 PinePhone.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Link: https://patch.msgid.link/20250706095804.50475-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Chen-Yu Tsai <wens@csie.org>