dts: overlay: Provide local-mac-address nodes as dts overlay

The 'local-mac-address' nodes are helpful for assessing if the LLDP like
example of DSA switch works correctly.

As those are used only for testing (or when user explicitly needs them),
those are added as DTS overlay to DSA example.

The 'local-mac-address' properties will be visible when one assess the
LLDP response from ip_k66f board (each LAN port has different IP
assigned).

Example output from 'tcpdump -v -i eth0':
-----------------------------------------
       Chassis ID TLV (1), length 7
          Subtype MAC address (4): 00:00:12:13:00:37 (oui Unknown)
        Port ID TLV (2), length 7
          Subtype MAC address (3): 00:00:12:13:00:37 (oui Unknown)
        Time to Live TLV (3), length 2: TTL 120s
        System Name TLV (5), length 13: ip_k66f LAN:3
        End TLV (0), length 0


Signed-off-by: Lukasz Majewski <lukma@denx.de>