Add support for TL-WA801ND v4

Add support for TL-WA801ND v4

This add support for TP-Link TL-WA801ND v4 (same as TL-WA801ND v3) :

Specification:

- System-On-Chip: Qualcomm Atheros QCA9533
- CPU/Speed: 650 MHz
- Flash-Chip: Winbond W25Q32BVSIG
- Flash size: 4096 KiB
- RAM: 32 MiB
- Wireless No1: SoC-integrated: QCA9533 2.4GHz 802.11bgn

Flash instructions:

1) To flash the image, rename the file
   openwrt-ar71xx-generic-tl-wa801nd-v4-squashfs-factory.bin
   to firmware.bin
2) Connect your device to the LAN port, then upload the firmware
   through web interface. It will try to download the image and
   flash it.

It can take up to 2-3 minutes to finish. When it reaches 100%, the
router will reboot itself.

Signed-off-by: Romain MARIADASSOU <roms2000@free.fr>
(backported from 02494fa35ca07f01598e8bddb14303486ba69ba4)