brcm47xx: explicitely mention MIPS to avoid confusion

And while at it, fix the nomenclature, BCM947xx/953xx refer to specific boards,
while BCM47xx/BCM53xx refer to chip identification number.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39841 3c298f89-4303-0410-b956-a3cf2f4a3e73