fpga: dfl: move dfl_device_id to mod_devicetable.h

fpga: dfl: move dfl_device_id to mod_devicetable.h

In order to support MODULE_DEVICE_TABLE() for dfl device driver, this
patch moves struct dfl_device_id to mod_devicetable.h

Some brief description for DFL (Device Feature List) is added to make
the DFL known to the whole kernel.

Reviewed-by: Tom Rix <trix@redhat.com>
Acked-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
Signed-off-by: Russ Weight <russell.h.weight@intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Link: https://lore.kernel.org/r/20210107043714.991646-5-mdf@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>