powerpc: Implement nvram ioctls

Add the powerpc-specific ioctls to the nvram module. This allows the nvram
module to replace the generic_nvram module.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>