ARM: dts: imx6qdl: Add missing phy-mode and fixed links

ARM: dts: imx6qdl: Add missing phy-mode and fixed links

The DSA framework has got more picky about always having a phy-mode
for the CPU port. Add a phy-mode based on what the SoC ethernet is
using. For RGMII mode, have the switch add the delays.

Additionally, the cpu label has never actually been used in the
binding, so remove it.

Lastly add a fixed-link node indicating the expected speed/duplex of
the link to the SoC.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>