MIPS: BCM47XX: Add board detection

Detect on which board this code is running based on some nvram
settings. This is needed to start board specific workarounds and
configure the leds and buttons which are on different gpios on every board.

This patches add some boards we have seen, but there are many more.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5840
Patchwork: https://patchwork.linux-mips.org/patch/5855/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>