ath79: add PoE passthrough switch for Ubiquiti Nanostation (XM/XW)

This adds the gpio switch to enable PoE passthrough on Ubiquiti
Nanostation (XM/XW).

Values are copied from the implementation in ar71xx.

GPIO values checked on:
- NanoStation M5 XW
- NanoStation M2 XM

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 317e98a5a2c6f1e2ac599ec530478669b965602c)

target/linux/ath79/base-files/etc/board.d/03_gpio_switches

@@ -30,6 +30,12 @@ tplink,archer-c25-v1)
 ubnt,nanostation-ac)
 	ucidef_add_gpio_switch "poe_passthrough" "PoE Passthrough" "3"
 	;;
+ubnt,nanostation-m)
+	ucidef_add_gpio_switch "poe_passthrough" "PoE Passthrough" "8"
+	;;
+ubnt,nanostation-m-xw)
+	ucidef_add_gpio_switch "poe_passthrough" "PoE Passthrough" "2"
+	;;
 ubnt,acb-isp)
 	ucidef_add_gpio_switch "poe_passthrough" "PoE Passthrough" "11"
 	;;