sfc: extend MTD support for newer hardware

The X2 family of NICs (based on the SFC9250) have additional
MTD partitions for firmware and configuration. This includes
partitions that are read-only.

The NICs also have extended versions of the NVRAM interface,
allowing more detailed status information to be returned.

Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>