dt: bindings: add documentation for zx2967 family reset controller

This patch adds dt-binding documentation for zx2967 family
reset controller.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Shawn Guo <shawnguo@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>