boards: arm64: intel_socfpga: Move boards to subdirectories

Adjust boards directory structure to SoC structure:

 `boards/intel/intel_socfpga_agilex_socdk` ->
     `boards/intel/intel_socfpga/agilex_socdk`
 `boards/intel/intel_socfpga_agilex5_socdk` ->
     `boards/intel/intel_socfpga/agilex5_socdk`

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>