arm64: dts: imx8mm-venice-gw790: Remove phy-mode from switch node

arm64: dts: imx8mm-venice-gw790: Remove phy-mode from switch node

Per microchip,ksz.yaml, phy-mode is not a valid property in the
top-level switch node.

phy-mode = "rgmii-id" is already passed in the CPU port switch (port@5).

Remove it from the top-level switch node to fix the following
schema warning:

switch@5f: Unevaluated properties are not allowed ('phy-mode' was unexpected)
from schema $id: http://devicetree.org/schemas/net/dsa/microchip,ksz.yaml

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>