net: thunderx: Use fwnode_get_mac_address()

Replace the custom implementation with fwnode_get_mac_address,
which works on both DT and ACPI platforms.

While here, replace memcpy() by ether_addr_copy().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>