sfc: Extend MTD driver for use with new NICs

In new NICs flash is managed by firmware and we will use high-level
operations on partitions rather than direct SPI commands.  Add support
for multiple MTD partitions per flash device and remove the direct
link between MTD and SPI devices.  Maintain a list of MTD partitions
in struct efx_nic.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>