ath79: add SUPPORTED_DEVICES to ubnt_nanostation-m-xw

ath79: add SUPPORTED_DEVICES to ubnt_nanostation-m-xw

The ar71xx images for the Ubiquiti NanoStation M (XW) devices use
"nanostation-m-xw" as the board name, but the ath79 images are only
compatible with the "nano-m-xw" board name, so sysupgrade complains.

By adding this additional supported device, sysuspgrade smoothly
upgrades from ar71xx to ath79.

Tested on a NanoStation M (XW) running OpenWrt ar71xx r10250-016d1eb.

Ref: https://github.com/openwrt/openwrt/pull/2418
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
[removed duplicate DEVICE_VARIANT, removed uneeded nano-m-xw support]
Signed-off-by: Petr Štetiar <ynezz@true.cz>