dt-bindings: uniphier: move cache-uniphier.txt to vendor directory

Now, the Socionext vendor directory is available at
Documentation/devicetree/bindings/arm/socionext/

Move cache-uniphier.txt over to it.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>