net: Add eth_platform_get_mac_address() helper.

net: Add eth_platform_get_mac_address() helper.

BugLink: http://bugs.launchpad.net/bugs/1547674
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 <davem@davemloft.net>
(cherry picked from net-next commit c7f5d105495a38ed09e70d825f75d9d7d5407264)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>