Documentation: DT bindings: add doc for Altera's SoCFPGA platform

Document "altr,socfpga-cyclone5", "altr,socfpga-arria5", and
"altr,socfpga-arria10".

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Rob Herring <robh@kernel.org>