ath79/mikrotik: don't use mtd-mac-address in DTS

ath79/mikrotik: don't use mtd-mac-address in DTS

As evidenced here[1] the device MAC address can be stored at a random
offset in the hard_config partition. Rely on sysfs to update the MAC
address correctly.

To match sticker and vendor OS behavior, WAN MAC is set to the device
base MAC and LAN MAC is incremented from that.

Note: this will trigger a harmless kernel message during boot:
ag71xx 19000000.eth: invalid MAC address, using random address

There is no clean workaround to prevent this message from being emitted.

[1] https://github.com/openwrt/openwrt/pull/2850#issuecomment-610809021

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>