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

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>