MIPS: BCM47XX: handle different nvram sizes

The old code just worked for nvram with a size of 0x8000 bytes. This
patch adds support for reading nvram from partitions of 0xF000 and
0x10000 bytes.

There is just 32KB space for the nvram, but most devices do not use the
full size and this code reads the first 32KB in that case and prints a
warning.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4743/


Signed-off-by: John Crispin <blogic@openwrt.org>