ath79: create DTSI for ar9341 TP-Link devices

ath79: create DTSI for ar9341 TP-Link devices

This patch creates a shared DTSI for the TP-Link devices based
on ar9341 as those share a lot of definitions.

While at it, change from gpio-keys-polled to gpio-keys, remove
unused pll-data and remove some inherited stuff, too.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>