ramips: convert TP-Link MT7620 boards to tpt trigger

ramips: convert TP-Link MT7620 boards to tpt trigger

This converts all MediaTek MT7620 boards from TP-Link to use the now
supported WiFi throughput LED trigger. This way, the LED state now
covers all VAPs regardless of their name.

Also align all single-WiFi LEDs to represent the state of the 2.4GHz
radio. This was not always the case previously, as later-added support
for the MT7610 altered the phy probing order.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 1e7c6381f0058e49caab8de54eaad4862732f95d)