dt-bindings: reset: oxnas: Update for OX820

Add new compatible string for reset and sys-ctrl for the Oxford
Semiconductor OX820 Support.
Drop the OX810SE indices since they moved in a dedicated include file.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>