lantiq: dts: vr9: Add missing properties to the CPU port on the switch

lantiq: dts: vr9: Add missing properties to the CPU port on the switch

The CPU port should define the phy-mode and and a PHY phandle or
fixed-link to indicate how the CPU port is connected to the SoC's
Ethernet controller. On xRX200 this is all internal connection, so use
phy-mode = "internal" along with a fixed-link that matches the
definition inside &eth0.

Linux 6.0 shows a warning since upstream commit e09e9873152e3f ("net:
dsa: make phylink-related OF properties mandatory on DSA and CPU
ports"). when these properties are missing. Adding the properties
before OpenWrt is updated to Linux 6.0 is harmless.

Suggested-by: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>