MIPS: BCM47XX: add cpu-feature-overrides.h

The BCM47XX SoC code missed a cpu-feature-overrides.h header file, this
patch adds it. This code supports a long line of SoCs with different
features so for some features we still have to rely on the runtime
detection.

This was crated by checking the features of a BCM4712, BCM4704,
BCM5354, BCM4716 and BCM4706 SoC and then tested on these SoCs. There
are some SoCs missing but I hope they do not have any more or less
features.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6289/