brcm63xx: add initial support for Netgear EVG2000

brcm63xx: add initial support for Netgear EVG2000

This patch adds support for the Netgear EVG2000 VoIP Gateway to the
bcm63xx targets.

This device was not sold to the general public, but rather is/was
provided by telcos to customers in Sweden, Australia, Singapore and
other parts of asia.

Known issues:
 - Unable to detect 53115 switch. This appear to be a problem with
probing for the PHY using MDIO and results in error 5. Doesn't seem to
be a problem with the configuration, and could use someone with
experience to have a look at it.
 - Uses the b43 driver as using the OpenWRT/LEDE broadcom-wl driver
fails to load the firmware for the 4322, so 802.11n is not supported.

More info on the device and the research can be found at:
https://wiki.openwrt.org/toh/netgear/evg2000
https://wikidevi.com/wiki/Netgear_EVG2000
https://github.com/Xotic750/mirror-lede/tree/evg2000
https://forum.openwrt.org/viewtopic.php?id=63950

Signed-off-by: Graham Fairweather <xotic750@gmail.com>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>