fix generate_bl31_node failed to get list of *.bin files

[thanhlev]

This PR is subject to fixing the issue of failure to build u-boot for the rk3568-rock-3a board. Issue:

./build/mk-uboot.sh rk3568-rock-3a

---
FATAL ERROR: Couldn't open "./09:46": No such file or directory
./tools/mkimage: Can't read u-boot.itb.tmp: Invalid argument
make: *** [Makefile:1051: u-boot.itb] Error 255
 MAKE UBOOT IMAGE FAILED.

Root cause:

• The function generate_bl31_node was failed to get the file name of bl31_0x*.bin due to incorrect index for awk.

ls -l bl31_0x*.bin | sort --key=5 -nr | awk '{ print $9 }'
09:46
09:46
09:46
09:46
09:46

Solution:

• Correct the index to get the result in awk

ls -l bl31_0x*.bin | sort --key=5 -nr | awk '{ print $10 }'
bl31_0x00040000.bin
bl31_0x00068000.bin
bl31_0xfdcd0000.bin
bl31_0xfdcc9000.bin
bl31_0x00066000.bin