MIPS: BCM63XX: move nvram functions into their own file

Refactor nvram related functions into its own unit for easier expansion
and exposure of the values to other drivers.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/4516
Signed-off-by: John Crispin <blogic@openwrt.org>