net: usb: asix: allow optionally getting mac address from device tree

For Embedded use where e.g. AX88772B chips may be used without external
EEPROMs the boot loader may choose to pass the MAC address to be used
via device tree. Therefore, allow for optionally getting the MAC
address from device tree data e.g. as follows (excerpt from a T30 based
board, local-mac-address to be filled in by boot loader):

/* EHCI instance 1: USB2_DP/N -> AX88772B */
usb@7d004000 {
	status = "okay";
	#address-cells = <1>;
	#size-cells = <0>;
	asix@1 {
		reg = <1>;
		local-mac-address = [00 00 00 00 00 00];
	};
};

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>