ath79: add support for ALFA Network AP121F

ath79: add support for ALFA Network AP121F

This commit ports support for the ALFA Network AP121F, a pocket-size
router with 1 Ethernet and 2.4 GHz WiFi based on the AR9331 SoC, to the
ath79 target (it was already supported in ar71xx; see commit 0c6165d2
for more details).

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
[pepe2k@gmail.com: fixed GPIO polarity, included USB support, changed
DTS nodes order, moved WLAN LED trigger define to DTS, made U-Boot env
partition writable]
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>