target/riscv: Update with latest encoding from riscv-opcodes

This gets us a clearly labeled BSD-3-Clause header, which should be
compatible with OpenOCD and Fedora, and also make it clear what the
license actually is.

See riscv-collab/riscv-openocd#710,
riscv-collab/riscv-openocd#713, and
riscv-collab/riscv-openocd#717.

Change-Id: I992b4f3bb230edb9f281e2278dd41c712098ed4c
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7084
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>