ath79: add gpio4 pinmux on TL-WR841N/ND v8, WR842N v2, MR3420 v2

ath79: add gpio4 pinmux on TL-WR841N/ND v8, WR842N v2, MR3420 v2

This adds a pinmux to the shared DTSI for TP-Link TL-WR841N/ND v8,
TL-WR842N v2 and TL-MR3420 v2. It is supposed to be the equivalent
of:

/* config gpio4 as normal gpio function */
ath79_gpio_output_select(TL_MR3420V2_GPIO_USB_POWER,AR934X_GPIO_OUT_GPIO);

This allows to enable USB power on these devices.

While at it, move the jtag_disable_pins to &gpio node and remove the
redundant status=okay there.

Tested on TP-Link TL-WR842N v2.

Fixes: FS#2753
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Armin Fuerst <armin@fuerst.priv.at>