ath79: add support for Linksys EA4500 v3

ath79: add support for Linksys EA4500 v3

Add support for the Linksys EA4500 v3 wireless router

Hardware
--------
SoC:    Qualcomm Atheros QCA9558
RAM:    128M DDR2 (Winbond W971GG6KB-25)
FLASH:  128M SPI-NAND (Spansion S34ML01G100TFI00)
WLAN:   QCA9558 3T3R 802.11 bgn
        QCA9580 3T3R 802.11 an
ETH:    Qualcomm Atheros QCA8337
UART:   115200 8n1, same as ea4500 v2
USB:	1 single USB 2.0 host port
BUTTON: Reset - WPS
LED:    1x system-LED
        LEDs besides the ethernet ports are controlled
        by the ethernet switch

MAC Address:
 use        address(sample 1)    source
 label      94:10:3e:xx:xx:6f   caldata@cal_macaddr
 lan        94:10:3e:xx:xx:6f   $label
 wan        94:10:3e:xx:xx:6f   $label
 WiFi4_2G   94:10:3e:xx:xx:70   caldata@cal_ath9k_soc
 WiFi4_5G   94:10:3e:xx:xx:71   caldata@cal_ath9k_pci

Installation from Serial Console
------------

1. Connect to the serial console. Power up the device and interrupt
   autoboot when prompted

2. Connect a TFTP server reachable at 192.168.1.0/24
   (e.g. 192.168.1.66) to the ethernet port. Serve the OpenWrt
   initramfs image as "openwrt.bin"

3. To test OpenWrt only, go to step 4 and never execute step 5;
   To install, auto_recovery should be disabled first, and boot_part
   should be set to 1 if its current value is not.

   ath> setenv auto_recovery no
   ath> setenv boot_part 1
   ath> saveenv

4. Boot the initramfs image using U-Boot

   ath> setenv serverip 192.168.1.66
   ath> tftpboot 0x84000000 openwrt.bin
   ath> bootm

5. Copy the OpenWrt sysupgrade image to the device using scp and
   install it like a normal upgrade (with no need to keeping config
   since no config from "previous OpenWRT installation" could be kept
   at all)

   # sysupgrade -n /path/to/openwrt/sysupgrade.bin

Note: Like many other routers produced by Linksys, it has a dual
      firmware flash layout, but because I do not know how to handle
      it, I decide to disable it for more usable space. (That is why
      the "auto_recovery" above should be disabled before installing
      OpenWRT.) If someone is interested in generating factory
      firmware image capable to flash from stock firmware, as well as
      restoring the dual firmware layout, commented-out layout for the
      original secondary partitions left in the device tree may be a
      useful hint.

Installation from Web Interface
------------

1. Login to the router via its web interface (default password: admin)

2. Find the firmware update interface under "Connectivity/Basic"

3. Choose the OpenWrt factory image and click "Start"

4. If the router still boots into the stock firmware, it means that
   the OpenWrt factory image has been installed to the secondary
   partitions and failed to boot (since OpenWrt on EA4500 v3 does not
   support dual boot yet), and the router switched back to the stock
   firmware on the primary partitions. You have to install a stock
   firmware (e.g. 3.1.6.172023, downloadable from
   https://www.linksys.com/support-article?articleNum=148385 ) first
   (to the secondary partitions) , and after that, install OpenWrt
   factory image (to the primary partitions). After successful
   installation of OpenWrt, auto_recovery will be automatically
   disabled and router will only boot from the primary partitions.

Signed-off-by: Edward Chow <equu@openmail.cc>
(cherry picked from commit 50f727b7737d118f7d44986181e305af0624c41d)