dt-bindings: fpga: add authenticate-fpga-config property

commit 55d29b377823ed7066e6e179e90fd4608690dd40 from
https://github.com/altera-opensource/linux-socfpga.git

Add authenticate-fpga-config property for FPGA bitstream authentication,
which makes sure a signed bitstream has valid signatures.

Signed-off-by: Richard Gong <richard.gong@intel.com>
Signed-off-by: Liwei Song <liwei.song@windriver.com>