ARC: reset: add missing DT binding documentation for HSDKv1 reset driver

When applying the original patch [1], the DT binding docs were lost.
This patch adds them back.

[1] https://patchwork.kernel.org/patch/9852997/

Fixes: e0be864f

 ("ARC: reset: introduce HSDKv1 reset driver")
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>