ar71xx: Add support for TP-Link MR6400

You can flash via tftp recovery (serve factory image as /mr6400_tp_recovery.bin
on 192.168.0.66/24, connect to any ethernet port and power on device while
holding the reset button). Flashing via OEM web interface does not work.

Hardware Specification (v1.0 EU):
 - SoC: QCA9531
 - Flash: Winbond W25Q64FV (8MiB)
 - RAM: EtronTech EM6AB160TSE-5G (64MiB)
 - Wireless: SoC platform only (2.4GHz b/g/n, 2x internal antenna)
 - Ethernet: 2NIC (3x100M + 1x100M)
 - WWAN: TP-LINK LTE MODULE (2x external detachable antenna)
 - Power: DC 12V 1A

Signed-off-by: Filip Moc <lede@moc6.cz>

target/linux/ar71xx/base-files/lib/ar71xx.sh

@@ -309,6 +309,9 @@ tplink_board_detect() {
 	"49000002")
 		model="TP-Link TL-WDR4900"
 		;;
+	"640000"*)
+		model="TP-Link TL-MR6400"
+		;;
 	"65000002")
 		model="TP-Link TL-WDR6500"
 		;;
@@ -1016,6 +1019,9 @@ ar71xx_board_detect() {
 	*"TL-MR3420 v2")
 		name="tl-mr3420-v2"
 		;;
+	*TL-MR6400)
+		name="tl-mr6400"
+		;;
 	*"TL-WA701ND v2")
 		name="tl-wa701nd-v2"
 		;;

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -398,6 +398,7 @@ platform_check_image() {
 	tl-mr3220|\
 	tl-mr3420-v2|\
 	tl-mr3420|\
+	tl-mr6400|\
 	tl-wa701nd-v2|\
 	tl-wa7210n-v2|\
 	tl-wa750re|\