net: gem: Workaround gmii2rgmii bridge DT node issue

For configurations with gmii2rgmii and external phy the DT nodes link
should be gem->gmii2rgmii->phy. But due to limitation in Linux driver
the DT is mentioned as gem->phy and gmii2rgmii->phy as shown in below DT.

ethernet@ff0c0000 {
	compatible = "cdns,zynqmp-gem\0cdns,gem";
	status = "okay";
	interrupt-parent = <0x04>;
	interrupts = <0x00 0x3b 0x04 0x00 0x3b 0x04>;
	reg = <0x00 0xff0c0000 0x00 0x1000>;
	clock-names = "pclk\0hclk\0tx_clk\0rx_clk\0tsu_clk";
	#address-cells = <0x01>;
	#size-cells = <0x00>;
	#stream-id-cells = <0x01>;
	iommus = <0x0d 0x875>;
	power-domains = <0x0c 0x1e>;
	clocks = <0x03 0x1f 0x03 0x69 0x03 0x2e 0x03 0x32 0x03 0x2c>;
	phy-handle = <0x0e>;
	phy-mode = "gmii";
	xlnx,ptp-enet-clock = <0x00>;
	local-mac-address = [ff ff ff ff ff ff];
	phandle = <0x4d>;

	mdio {
		#address-cells = <0x01>;
		#size-cells = <0x00>;
		phandle = <0x4e>;

		ethernet-phy@1 {
			reg = <0x01>;
			rxc-skew-ps = <0x708>;
			txc-skew-ps = <0x708>;
			phandle = <0x0e>;
		};

		gmii_to_rgmii_0@8 {
			compatible = "xlnx,gmii-to-rgmii-1.0";
			phy-handle = <0x0e>;
			reg = <0x08>;
			phandle = <0x4f>;
		};
	};
};

Since same DT is used in Linux and U-Boot we need to workaround this
issue by using the gmii2rgmii node which points to phy and we should
ignore the gem pointing to phy directly.

Do this workaround by updating priv->phydev->node value with
priv->phy_of_node only if it is not valid node.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Link: https://lore.kernel.org/r/641eb13425ffe80e0743f60cf90d0f940577b9e9.1642162085.git.michal.simek@xilinx.com