ar71xx: fix reset key for TP-Link TL-WR802N V1/V2

ar71xx: fix reset key for TP-Link TL-WR802N V1/V2

During porting support for this router to ath79 target
it was discovered that GPIO mapping was incorrect (GPIO11 active high).
Correct mapping for both V1 and V2 is GPIO12 active low.

Default configuration from GPL source for V2 explicitly states this, and
this was confirmed experimentally on ath79 by looking on
/sys/kernel/debug/gpio. Correctness of this was also validated for V1 by
cross-flashing vendor firmware for V1 on V2 hardware, in which reset
button also worked.

Fix it.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
[slightly adjust commit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>