ar71xx: add support for P&W R602N and CPE505N

ar71xx: add support for P&W R602N and CPE505N

P&W (full name: Shenzhen Progress&Win Technologies) R602N (could be also
labeled as R602F, R602, etc.) is a simple N300 router with 5-port
10/100 Mbps switch, non-detachable antennas and USB.

CPE505 is an outdoor CPE with PoE support and detachable antennas.

Both devices are based on Qualcomm/Atheros QCA9531 v2.

Common specification:

- 650/597/216 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 16 MB of FLASH
- UART (J2) header on PCB

R602N specification:

- 5x 10/100 Mbps Ethernet
- 1x USB 2.0
- 2T2R 2.4 GHz with external LNA and PA (SE2576L), up to 28 dBm
- 2x external, non-detachable antennas
- 7x LED, 1x button

CPE505N specification:

- 2x 10/100 Mbps Ethernet (both ports support passive PoE 12-24 V)
- 2T2R 2.4 GHz with external LNA and PA (SKY65174-21), up to 30 dBm
- 2x external, detachable antennas (RP-SMA connectors)
- 1x RGB LED, 2x LEDs (in RJ45 sockets), 1x button

Flash instructions:

It seems that there are many different versions of the firmware which
these devices are shipped with. The generic/standard one is based on
some modified OpenWrt and LEDE firmware can be flashed directly from
vendor's webgui or with sysupgrade (root password is "admin123").

Before flashing, make sure (use "fw_printenv") that the kernel load
address in your device is set to "0x9f050000" (bootcmd variable is
"bootm 0x9f050000"). If your device uses different load address, you
should first change it, under vendor's firmware, with command:

fw_setenv bootcmd "bootm 0x9f050000 || bootm OLD_ADDRESS"

Where OLD_ADDRESS is previous kernel load address (in CPE505 version
I got access to, it was "0x9fe80000"). This will allow you to use
both the vendor's and LEDE firmware.

If version of your device contains empty U-Boot environment (you will
get information about this after issuing "fw_printenv"), you should
use U-Boot, serial line access and TFTP to perform firmware upgrade:

1. tftp 0x80060000 lede-ar71xx-generic-...-squashfs-sysupgrade.bin
2. erase 0x9f050000 +$filesize
3. cp.b $fileaddr 0x9f050000 $filesize
4. setenv bootcmd "bootm 0x9f050000 || bootm OLD_ADDRESS"
5. saveenv && reset

These devices contain also web recovery mode inside U-Boot. It can be
started with pressing the reset button for around 3 seconds just after
the device powerup. Web recovery panel is available on "192.168.10.9"
and to be able to use it, IP on your PC must be set to "192.168.10.10".

Make sure to change kernel load address before using recovery mode or
the U-Boot will not be able to load LEDE firmware.

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