fpga: region: Add fpga-region property 'encrypted-fpga-config'

Add fpga-region property to allow passing the fact that the bitstream is
encrypted to the fpga-region and ultimately to the low-level driver.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>