dt-bindings: arm: zte: Move sysctrl bindings to their own doc

In preparation to convert board-level bindings to json-schema, move
various misc SoC bindings out to their own file.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Jun Nie <jun.nie@linaro.org>
Cc: Baoyou Xie <baoyou.xie@linaro.org>
Cc: devicetree@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>