tg3: optionally use eth_platform_get_mac_address() to get mac address

This function will try to determine the mac address via the devicetree,
or via an architecture-specific method (e.g. a PROM on SPARC).

The SPARC-specific code in this driver (#ifdef SPARC) did exactly this,
and is therefore removed.

Note that you can now specify the tg3 mac address via the devicetree,
on any platform, not just SPARC:

Devicetree example:
(see Documentation/devicetree/bindings/pci/pci.txt)

&pcie {
	host@0 {
		#address-cells = <3>;
		#size-cells = <2>;
		reg = <0 0 0 0 0>;
		bcm5778: bcm5778@0 {
			reg = <0 0 0 0 0>;
			mac-address = [CA 11 AB 1E 10 01];
		};
	};
};

Signed-off-by: Sven Van Asbroeck <svendev@arcx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>