MIPS: BCM47xx: Add NVRAM support devices

When trying to netboot a Linksys WRT54GS WLAN router, the bootup fails,
because of following error message:

...
[    0.424000] b44: b44.c:v2.0
[    0.424000] b44: Invalid MAC address found in EEPROM
[    0.432000] b44 ssb0:1: Problem fetching invariants of chip,aborting
[    0.436000] b44: probe of ssb0:1 failed with error -22
...

The router uses a CFE bootloader, but most of the needed environment
variables for network card initialization, are not available from CFE
via printenv and even though not via cfe_getenv().
The required environment variables are saved in a special partition
in flash memory. The attached patch implement nvram_getenv and enables
bootup via NFS root on my router.

Most of the patch is extracted from the OpenWrt subversion repository and
stripped down and cleaned up to just fix this issue.

[Ralf: sorted out header file inclusions.  Lots of unneded headers and such
that should have been included.]

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Reviewed-by: Phil Sutter <phil@nwl.cc>
To: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/1359/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>