net/sfc/base: import MCDI implementation

net/sfc/base: import MCDI implementation

Implement interface to talk to NIC management CPU. Provide
helpers to fill in MCDI requests, execute it and process
received response.

MCDI request is prepared in either PCI BAR mapped memory
(SFN5xxx/SFN6xxx) or DMA-mapped memory (SFN7xxx/SFN8xxx) and,
doorbell is pressed (memory-mapped register) to execute it.

Events about MCDI completion are delivered to house-keeping
event queue, but usage of these events is optional and MCDI
buffer may be simply polled waiting for completion
indication set.

From Solarflare Communications Inc.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>