README: Update license information

With the Kendryte K210 platform code cleanup, none of the code copied
from Kendryte standalone SDK remains and this platform code is now
entirely licensed under OpenSBI BSD-2-clause license. Update the
README.md and ThirdPartyNotices.md files to reflect this.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>