ath79: fix/improve LED control for TL-WPA8630 v1/v2

ath79: fix/improve LED control for TL-WPA8630 v1/v2

The TL-WPA8630 v1 and v2 have the same LED Control GPIO configuration
according to the TP-Link GPL sources. Set the GPIO to output to make
it work and set to Active Low. It defaults to LEDs on at bootup.

To turn all LEDs off:
  echo 0 > /sys/class/gpio/tp-link\:led\:control/value
To turn all LEDs on:
  echo 1 > /sys/class/gpio/tp-link\:led\:control/value

Change the "LED" button from BTN_0 to KEY_LIGHTS_TOGGLE to match other
devices and the button guide, and to reduce the number of unintuitive
"BTN_X" inputs.

Fixes: ab74def0db93 ("ath79: add support for TP-Link TL-WPA8630P v2")
Signed-off-by: Joe Mullally <jwmullally@gmail.com>
[shorten commit title, minor commit message adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>