arm64: dts: freescale: ZII: Add missing phy-mode

The DSA framework has got more picky about always having a phy-mode
for the CPU port. The imx8mq Ethernet is being configured to RMII. Set
the switch phy-mode based on this.

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

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