samples: subsys: shell: Custom configs for intel_socfpga_agilex* boards

The intent of this change is to add custom shell configurations for
intel_socfpga_agilex* based boards. As of now, configurations are
added for 'intel_socfpga_agilex5_socdk' board.

Signed-off-by: Girisha Dengi <girisha.dengi@intel.com>