ARM: socfpga: add bindings document for fpga bridge drivers

Add bindings documentation for Altera SOCFPGA bridges:
 * fpga2sdram
 * fpga2hps
 * hps2fpga
 * lwhps2fpga

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Signed-off-by: Matthew Gerlach <mgerlach@altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>