bcm53xx: add initial support for ARM based BCM47XX and BCM53XX SoCs

This was only tested on a Netgear R6250, but it could also work on 
other devices when the correct images are generated.
It is only possible to boot into a ram disk, no Ethernet, Wifi, flash, 
USB is supported.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt@37268 3c298f89-4303-0410-b956-a3cf2f4a3e73