Revert "ath79-generic: remove Ubiquiti AirMax targets (#3067)"

This reverts commit a0f5b4e9.

Issues with the flash should be fixed as of OpenWRT commit
c55aaa7c9a ("ath79: generic: disable SPI-NOR write protect
unconditionally").
https://github.com/openwrt/openwrt/commit/c55aaa7c9a98e7c0a5e1ea8293a534dc5b395cf3