dt-bindings: net: dsa: microchip: add missing CPU port phy-mode to example

The ksz_switch_chips[] element for KSZ9477 says that port 5 is an xMII
port and it supports speeds of 10/100/1000. The device tree example does
declare a fixed-link at 1000, and RGMII is the only one of those modes
that supports this speed, so use this phy-mode.

The microchip,ksz8565 compatible string is not supported by the
microchip driver, however on Microchip's product page it says that there
are 5 ports, 4 of which have internal PHYs and the 5th is an
MII/RMII/RGMII port. It's a bit strange that this is port@6, but it is
probably just the way it is. Select an RGMII phy-mode for this one as
well.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>