MIPS: BCM47XX: Add support for Netgear WNR3500L v2

Add support for the Netgear WNR3500L v2 router based on the BCM47186
chipset and supporting 802.11n Wi-Fi.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>