arm: socfpga: Restructure FPGA driver in the preparation to support A10

arm: socfpga: Restructure FPGA driver in the preparation to support A10

Move FPGA driver which is Gen5 specific code into Gen5 driver file
and keeping common FPGA driver intact. All the changes are still keeping
in driver/fpga/ and no functional change. Subsequent patch would move
FPGA manager driver from arch/arm into driver/fpga/.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>
Reviewed-by: Dinh Nguyen <dinguyen@kernel.org>