dt-bindings: Document the DesignWare IP reset bindings

This adds documentation of device tree bindings for the
DesignWare IP reset controller.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Luis Oliveira <luis.oliveira@synopsys.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>