memcpy stuck

[0xWilliamWang]

(I found I coludn't open issue, so created this PR.)

1. I was planning to run keystone v1 on hifive unleashed development board and got stuck on a memcpy call.
2. The 358 line memcpy call does not have an infinite loop, it just executes many times and finally gets stuck.
3. After about a minute, it got stuck and didn't execute 359 line
4. Below is the log running on hifive unleashed development board. The associated commit id is 1ed04b4

   SiFive FSBL:       2021-08-27-1ed04b4
   Using FSBL DTB
   first enter memcpy
   in memcpy
   in memcpy
   ...
   

5. I changed the memcpy param and it can work. The associated commit id is e229e99

   SiFive FSBL:       2021-08-27-e229e99
   Using FSBL DTB
   first enter memcpy
   first exit memcpy
   HiFive-U serial #: 0000032e
   Loading boot payload................................
   
   
   OpenSBI v0.8
   ____                    _____ ____ _____
   / __ \                  / ____|  _ \_   _|
   | |  | |_ __   ___ _ __ | (___ | |_) || |
   | |  | | '_ \ / _ \ '_ \ \___ \|  _ < | |
   | |__| | |_) |  __/ | | |____) | |_) || |_
   \____/| .__/ \___|_| |_|_____/|____/_____|
         | |
         |_|
   
   Platform Name             : sifive,hifive-unleashed-a00
   Platform Features         : timer,mfdeleg
   ...
   

6. I am not very familiar with fsbl dev, is there a better solution? (I think there might be dtb_target, dtb pointers overlap)
7. My build env:

   ➜  freedom-u540-c000-bootloader-keystone git:(probe-memcpy) ✗ riscv64-unknown-elf-gcc --version
   riscv64-unknown-elf-gcc (GCC) 10.2.0
   Copyright (C) 2020 Free Software Foundation, Inc.
   This is free software; see the source for copying conditions.  There is NO
   warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
   
   ➜  freedom-u540-c000-bootloader-keystone git:(probe-memcpy) ✗ uname -a
   Linux ToolMan 5.4.0-80-generic #90~18.04.1-Ubuntu SMP Tue Jul 13 19:40:02 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
   ➜  freedom-u540-c000-bootloader-keystone git:(probe-memcpy) ✗ git log -1 --oneline 
   e229e99 (HEAD -> probe-memcpy, github/probe-memcpy) modify memcpy param type
   ➜  freedom-u540-c000-bootloader-keystone git:(probe-memcpy) ✗