arm64: dts: rockchip: fix graph_port warning on rk3399 bob kevin and excavator

Ports are described by child 'port' nodes contained in the device node.
'ports' is optional and is used to group all 'port' nodes which is not
the case here.

This patch fixes the following warnings:

arch/arm64/boot/dts/rockchip/rk3399-gru-bob.dts:25.9-29.5: Warning (graph_port): /edp-panel/ports: graph port node name should be 'port'
arch/arm64/boot/dts/rockchip/rk3399-gru-kevin.dts:46.9-50.5: Warningi (graph_port): /edp-panel/ports: graph port node name should be 'port'
arch/arm64/boot/dts/rockchip/rk3399-sapphire-excavator.dts:94.9-98.5: Warning (graph_port): /edp-panel/ports: graph port node name should be 'port'

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>