fpga: fpga-region: fix kernel-doc formatting issues

fpga: fpga-region: fix kernel-doc formatting issues

To fix below kernel-doc warnings this patch does the following
->Replaced Return\Returns with 'Return:' keyword.
->Added 'Return' description For __init of_fpga_region_init()' API.
->Added description for 'child_regions_with_firmware()' API.

warning: No description found for return value of
'of_fpga_region_find'.
warning: No description found for return value of
'of_fpga_region_get_bridges'.
warning: missing initial short description on line:
* child_regions_with_firmware
warning: No description found for return value of
'child_regions_with_firmware'.
warning: No description found for return value of
'of_fpga_region_notify_pre_apply'.
warning: No description found for return value of
'of_fpga_region_notify'.
warning: No description found for return value of
'of_fpga_region_init'.

Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Link: https://lore.kernel.org/r/20220423170235.2115479-6-nava.manne@xilinx.com
Signed-off-by: Xu Yilun <yilun.xu@intel.com>