dt-bindings: reset: imx7: Document usage on i.MX8MP SoC

The driver now supports i.MX8MP, so update bindings accordingly.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>