ramips: fix flash layout for TP-Link TL-WR841N v14

ramips: fix flash layout for TP-Link TL-WR841N v14

The config partition was missing from the flash layout of the device.

Although the stock firmware resets a corrupted config partition to the
default values, the TFTP flash with an image bigger than 0x3d0000 will
truncate the image as the bootloader only copies 0x3d0000 bytes to flash
during TFTP flashing.

Fixed by adding the config partition and shrinking the firmware
partition.

Fixes: 3fd97c522bb7 ("ramips: add support for TP-Link TL-WR841n v14")
Signed-off-by: Alexander Müller <donothingloop@gmail.com>