spl: Setup default value for OF_LIST

OF_LIST can't remain empty that's why setup it up to default DTB.

If it is empty u-boot.img is created without FDT partition:
For example:
  ./tools/mkimage -f auto -A arm -T firmware -C none -O u-boot -a
0x8000000 -e 0 -n "U-Boot 2016.05-rc3 ..." -E -b  -d u-boot-nodtb.bin u-boot.img
Can't set 'timestamp' property for '' node (FDT_ERR_NOSPACE)
FIT description: Firmware image with one or more FDT blobs
Created:         Wed May  4 15:02:52 2016
 Image 0 (firmware@1)
  Description:  U-Boot 2016.05-rc3-00080-gff2e12ae22a8-dirty for zynqmp
board
  Created:      Wed May  4 15:02:52 2016
  Type:         Firmware
  Compression:  uncompressed
  Data Size:    unavailable
  Architecture: ARM
  Load Address: 0x08000000
 Default Configuration: 'conf@1'
 Configuration 0 (conf@1)
  Description:  unavailable
  Kernel:       unavailable

And then image like this doesn't contain description and link to FDT and
can't boot.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>