dt-bindings: soc: remove obsolete zte zx header

The zte zx platform had been removed in commit 89d4f98a ("ARM: remove
zte zx platform"), so this header is no longer needed.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jun Nie <jun.nie@linaro.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20210821030924.192-2-yuzenghui@huawei.com


Signed-off-by: Rob Herring <robh@kernel.org>