arm64: dts: uniphier: Add L2 cache node

Add a L2 cache node referenced from CPU nodes as the missing cache hierarchy
information because the following warning was issued.

  cacheinfo: Unable to detect cache hierarchy for CPU 0
  Early cacheinfo failed, ret = -2

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/20220913042321.4817-11-hayashi.kunihiko@socionext.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>