dt-bindings: arm: mediatek: apmixedsys: Convert to DT schema

Convert apmixedsys bindings to DT schema format. MT2701, MT7623 and
MT7629 device trees currently have the syscon compatible without
it being mentioned in the old DT bindings file which introduces
dtbs_check errors when converting to DT schema as-is, so
mediatek,mt2701-apmixedsys and mediatek,mt7629-apmixedsys are placed
in the last items list with the syscon compatible, and syscon is
added to the mediatek,mt7623-apmixedsys list.

Signed-off-by: Yassine Oudjana <y.oudjana@protonmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220424084647.76577-3-y.oudjana@protonmail.com