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 <[email protected]>