Document the Sparx5 reset device driver bindings

The driver uses a syscon and an IO range on sparx5 for access to
the reset control and the reset status.

Sparx5 will no longer use the existing Ocelot chip reset driver, but use
this new switch reset driver as it has the reset controller interface that
allows the first client to perform the reset on behalf of all the Sparx5
component drivers.
Signed-off-by: Steen Hegelund <steen.hegelund@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>