ar71xx: add correct named default wireless led by using platform leds

Instead of renaming the default wireless led attached to the wireless
chip, add a new led using the platform leds with the phy0tpt trigger
set in userspace.

When switching ar71xx to device tree, the same can be done by using the
build in GPIO controller and without adding new bindings.

Drop the now unused platform code.

Signed-off-by: Mathias Kresin <dev@kresin.me>