ar71xx: configure OBS4 line on TL-WR841N-v8/MR3420v2

It also fixes USB Power on MR3420v2.

This time we took the information from the source of GPL:
http://www.mail-archive.com/openwrt-devel@lists.openwrt.org/msg18970.html
Confirmed and tested:
https://dev.openwrt.org/ticket/13201#comment:41
Thanks to oguretsagressive for testing.

Signed-off-by: Dmytro <dioptimizer@gmail.com>

SVN-Revision: 37878

target/linux/ar71xx/files/arch/mips/ath79/mach-tl-wr841n-v8.c

@@ -132,8 +132,14 @@ static void __init tl_ap123_setup(void)
 	u8 *mac = (u8 *) KSEG1ADDR(0x1f01fc00);
 	u8 *ee = (u8 *) KSEG1ADDR(0x1fff1000);
 
-	/* Disable JTAG, enabling GPIOs 0-4 */
-	ath79_gpio_function_enable(AR934X_GPIO_FUNC_JTAG_DISABLE);
+	/* Disable JTAG, enabling GPIOs 0-3 */
+	/* Configure OBS4 line, for GPIO 4*/
+	ath79_gpio_function_setup(AR934X_GPIO_FUNC_JTAG_DISABLE,
+				 AR934X_GPIO_FUNC_CLK_OBS4_EN);
+
+	/* config gpio4 as normal gpio function */
+	ath79_gpio_output_select(TL_MR3420V2_GPIO_USB_POWER,
+				 AR934X_GPIO_OUT_GPIO);
 
 	ath79_register_m25p80(&tl_wr841n_v8_flash_data);