ipq40xx: re-add label MAC address for FritzBox 4040

ipq40xx: re-add label MAC address for FritzBox 4040

The MAC address of the GMAC is contained inside the CWMP-Account
number on the label.

The label MAC address alias was defined previously, but it has been
removed with the switch to IPQESS / DSA.

Restore the label MAC address alias.

Fixes: 27b441cbaf42 ("ipq40xx: drop ESSEDMA + AR40xx DTS nodes")
Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit b22d382ae4eaa1af42930115d91855f402314cac)
Signed-off-by: Tom Herbers <mail@tomherbers.de>
Link: https://github.com/openwrt/openwrt/pull/16459
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>