ARM: tegra: Implement cboot_get_ethaddr()

ARM: tegra: Implement cboot_get_ethaddr()

This function will attempt to look up an Ethernet address in the DTB
that was passed in from cboot. It does so by first trying to locate the
default Ethernet device for the board (identified by the "ethernet"
alias) and if found, reads the "local-mac-address" property. If the
"ethernet" alias does not exist, or if it points to a device tree node
that doesn't exist, or if the device tree node that it points to does
not have a "local-mac-address" property or if the value is invalid, it
will fall back to the legacy mechanism of looking for the MAC address
stored in the "nvidia,ethernet-mac" or "nvidia,ether-mac" properties of
the "/chosen" node.

The MAC address is then written to the default Ethernet device for the
board (again identified by the "ethernet" alias) in U-Boot's control
DTB. This allows the device driver for that device to read the MAC
address from the standard location in device tree.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>