ARM: dts: imx: add ethernet stop mode property

- Update the imx6qdl gpr property to define gpr register
  offset and bit in DT.
- Add imx6sx/imx6ul/imx7d ethernet stop mode property.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>