ath79: add support for TP-Link TL-WR740N v5

This adds support for the TP-Link TL-WR740N v5, a clone of the
v4 only with a different TPLINK_HWID. It was already supported
in ar71xx as well.

Specifications:

SOC: Atheros AR9331
CPU: 400MHz
Flash: 4 MiB
RAM: 32 MiB
WLAN: Atheros AR9330 bgn
Ethernet: 5 ports (100M)

Flashing instructions:

- Flash factory image from OEM WebUI:
  openwrt-ath79-tiny-tplink_tl-wr740n-v5-squashfs-factory.bin
- Sysupgrade from ar71xx image:
  openwrt-ath79-tiny-tplink_tl-wr740n-v5-squashfs-sysupgrade.bin

Signed-off-by: Jun Su <howard0su@gmail.com>
[commit title/message facelift, backport for 19.07]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit b9f4f1f97aeeec65a872c4c5b1a0528a69d16d7b)