fdtdec: Add fdtdec_set_ethernet_mac_address()

fdtdec: Add fdtdec_set_ethernet_mac_address()

This function can be used to set the local MAC address for the default
Ethernet interface in its device tree node. The default interface is
identified by the "ethernet" alias.

One case where this is useful is for devices that store their MAC
address in a custom location. Once extracted, board code can store the
MAC address in U-Boot's control DTB so that it will automatically be
used by the Ethernet uclass.

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