memory: dfl-emif: add the DFL EMIF private feature driver

This driver is for the EMIF private feature implemented under FPGA
Device Feature List (DFL) framework. It is used to expose memory
interface status information as well as memory clearing control.

The purpose of memory clearing block is to zero out all private memory
when FPGA is to be reprogrammed. This gives users a reliable method to
prevent potential data leakage.

[mdf@kernel.org: Fixed up ABI doc]

Reviewed-by: Tom Rix <trix@redhat.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Russ Weight <russell.h.weight@intel.com>
Link: https://lore.kernel.org/r/20210107043714.991646-9-mdf@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>