firmware: bcm47xx_nvram: Support small (0x6000 B) NVRAM partitions

Some old devices with 4 MiB flashes were using 0x1000 block size and
could use smaller (0x6000 bytes) flash partition for storing NVRAM
content. This adds support for reading NVRAM on Netgear WNR1000 V3.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/19005/
Signed-off-by: James Hogan <jhogan@kernel.org>