docs: update CONTRIBUTORS.md from git history

OpenSBI has recently gained a CONTRIBUTORS.md file as a way
to make contributions to OpenSBI more visible.  Add previous
contributors from the git history who have acked their
inclusion in the list to the file.

Signed-off-by: Karsten Merker <merker@debian.org>
Acked-by: Alistair Francis <alistair@alistair23.me>
Acked-by: Anup Patel <anup.patel@wdc.com>
Acked-by: Atish Patra <atish.patra@wdc.com>
Acked-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Damien Le Moal <damien.lemoal@wdc.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Acked-by: Shawn Chang <citypw@gmail.com>
Acked-by: Xiang Wang <wxjstz@126.com>