arm: dts: k3-am64: Fix CPSW3G ethernet

As MDIO driver does not support Driver Model, the
pinctrl settings in the MDIO node will not
be applied resulting in PHY not being detected.

To workaround this we add the MDIO pinctrl in
the CPSW3G node in the -u-boot.dtsi file.

Add the missing MDIO and RGMII pinctrl nodes in
k3-am642-r5-evm.dts

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Tom Rini <trini@konsulko.com>