As all level 2 and level 3 caches are unified, add required
cache-unified properties to fix warnings like:

  hi3660-hikey960.dtb: l2-cache0: 'cache-unified' is a required property

Link: https://lore.kernel.org/r/20230421223215.115666-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>