net: Add eth_platform_get_mac_address() helper.
A repeating pattern in drivers has become to use OF node information and, if not found, platform specific host information to extract the ethernet address for a given device. Currently this is done with a call to of_get_mac_address() and then some ifdef'd stuff for SPARC. Consolidate this into a portable routine, and provide the arch_get_platform_mac_address() weak function hook for all architectures to implement if they want. Signed-off-by: David S. Miller <firstname.lastname@example.org>