ar71xx: add support for COMFAST CF-E320N v2 and CF-E520N/CF-E530N

As we already have support for CF-E316N v2 and many devices from
this vendor look similar, the support was included in existing
mach-*.c file, with few cleanups and fixes.

All 3 devices are based on Qualcomm/Atheros QCA9531 v2.

COMFAST CF-E320N v2 is a ceiling mount AP with PoE support.
Short specification:

- 650/393/216 MHz (CPU/DDR/AHB)
- 2x 10/100 Mbps Ethernet, both with PoE support
- 64 MB of RAM (DDR2)
- 16 MB of FLASH
- 2T2R 2.4 GHz, up to 22 dBm
- 2x internal antennas
- 1x RGB LED, 1x button
- UART (J1), GPIO (J9) and USB (J2) headers on PCB
- external watchdog (Pericon Technology PT7A7514)

COMFAST CF-E520N/CF-E530N are in-wall APs with USB and PoE support.
They seem to have different only the front panel.
Short specification:

- 650/393/216 MHz (CPU/DDR/AHB)
- 2x 10/100 Mbps Ethernet, WAN with PoE support
- 1x USB 2.0 (in CF-E520N covered by panel, available on PCB)
- 32 MB of RAM (DDR2)
- 8 MB of FLASH
- 2T2R 2.4 GHz, up to 22 dBm
- 2x internal antennas
- 1x LED, 1x button
- UART (J1) headers on PCB

Flash instruction:

Original firmware is based on OpenWrt.
Use sysupgrade image directly in vendor GUI.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -182,6 +182,9 @@ platform_check_image() {
 	c-55|\
 	carambola2|\
 	cf-e316n-v2|\
+	cf-e320n-v2|\
+	cf-e520n|\
+	cf-e530n|\
 	dgl-5500-a1|\
 	dhp-1565-a1|\
 	dir-505-a1|\