ath79: add common definition for Device/tplink-v2

This adds a shared definition Device/tplink-v2 to common-tp-link.mk.
Though currently only one device in ath79 uses it, putting it in
the common file seems more organized. The definitions are based
on the implementation in ramips target, where a lot of devices
is using tplink-v2-* commands already.

The '-V "ver. 2.0"' suffix for Archer D50 v1 can be removed because
it's default in Build/tplink-v2-image anyway.

While at it, add TPLINK_HWREVADD and TPLINK_HVERSION to DEVICE_VARS,
which seems to have been overlooked when adding Archer D50 v1.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>