fpga: add device feature list support

Device Feature List (DFL) defines a feature list structure that creates
a linked list of feature headers within the MMIO space to provide an
extensible way of adding features. This patch introduces a kernel module
to provide basic infrastructure to support FPGA devices which implement
the Device Feature List.

Usually there will be different features and their sub features linked into
the DFL. This code provides common APIs for feature enumeration, it creates
a container device (FPGA base region), walks through the DFLs and creates
platform devices for feature devices (Currently it only supports two
different feature devices, FPGA Management Engine (FME) and Port which
the Accelerator Function Unit (AFU) connected to). In order to enumerate
the DFLs, the common APIs required low level driver to provide necessary
enumeration information (e.g. address for each device feature list for
given device) and fill it to the dfl_fpga_enum_info data structure. Please
refer to below description for APIs added for enumeration.

Functions for enumeration information preparation:
 *dfl_fpga_enum_info_alloc
   allocate enumeration information data structure.

 *dfl_fpga_enum_info_add_dfl
   add a device feature list to dfl_fpga_enum_info data structure.

 *dfl_fpga_enum_info_free
   free dfl_fpga_enum_info data structure and related resources.

Functions for feature device enumeration:
 *dfl_fpga_feature_devs_enumerate
   enumerate feature devices and return container device.

 *dfl_fpga_feature_devs_remove
   remove feature devices under given container device.

Signed-off-by: Tim Whisonant <tim.whisonant@intel.com>
Signed-off-by: Enno Luebbers <enno.luebbers@intel.com>
Signed-off-by: Shiva Rao <shiva.rao@intel.com>
Signed-off-by: Christopher Rauer <christopher.rauer@intel.com>
Signed-off-by: Zhang Yi <yi.z.zhang@intel.com>
Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Wu Hao <hao.wu@intel.com>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>