arm64: dts: imx8mp-venice-gw74xx: fix ksz9477 cpu port

The CPU uplink port on the KSZ9477 is P5 not P6 - fix this.

Fixes: 7899eb6c

 ("arm64: dts: imx: Add i.MX8M Plus Gateworks gw7400 dts support")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>