fpga: dfl: expose feature revision from struct dfl_device

DFL device drivers have a common need for checking feature revision
information from the DFL header, as well as other common DFL information
like the already exposed feature id and type.

This patch exposes the feature revision information directly via the DFL
device data structure.

Since the DFL core code has already read the DFL header, this this patch
saves additional mmio reads from DFL device drivers too.

Acked-by: Wu Hao <hao.wu@intel.com>
Acked-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
Signed-off-by: Martin Hundebøll <mhu@silicom.dk>
Signed-off-by: Moritz Fischer <mdf@kernel.org>