ramips: tl-wr840n-v5: increase firmware partition for 4Mmtk layot

According to console log during TP-Link TL-WR840N v5 OEM firmware update
procedure 0x3e0000-0x3f0000 64kB "config" partition, which is used to store
router's configuration settings, is erased and recreated again during every
OEM firmware update procedure, thus does not contain any valuable factory data.
So it is conviniant to use this extra 64kB erase block for jffs overlay due
limited flash size on this device like it used on TP-Link's ar71xx boards.

Signed-off-by: Serg Studzinskii <serguzhg@gmail.com>

target/linux/ramips/dts/TL-WR840NV5.dts

@@ -65,13 +65,7 @@
 
 		partition@20000 {
 			label = "firmware";
-			reg = <0x20000 0x3c0000>;
-		};
-
-		partition@3e0000 {
-			label = "config";
-			reg = <0x3e0000 0x10000>;
-			read-only;
+			reg = <0x20000 0x3d0000>;
 		};
 
 		factory: partition@3f0000 {

target/linux/ramips/image/mt76x8.mk

@@ -112,7 +112,7 @@ TARGET_DEVICES += tl-wr840n-v4
 
 define Device/tl-wr840n-v5
   DTS := TL-WR840NV5
-  IMAGE_SIZE := 3840k
+  IMAGE_SIZE := 3904k
   DEVICE_TITLE := TP-Link TL-WR840N v5
   TPLINK_FLASHLAYOUT := 4Mmtk
   TPLINK_HWID := 0x08400005

tools/firmware-utils/src/mktplinkfw2.c

@@ -124,7 +124,7 @@ char md5salt_boot[MD5SUM_LEN] = {
 static struct flash_layout layouts[] = {
 	{
 		.id		= "4Mmtk",
-		.fw_max_len	= 0x3c0000,
+		.fw_max_len	= 0x3d0000,
 		.kernel_la	= 0x80000000,
 		.kernel_ep	= 0x80000000,
 		.rootfs_ofs	= 0x140000,