arm64: dts: imx: add missing cache properties

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

  imx8dxl-evk.dtb: l2-cache0: 'cache-unified' is a required property

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>