target/riscv: Update debug_defines.h.

target/riscv: Update debug_defines.h.

This one doesn't have the license in there, which means now it's
acceptable to GPLv2 again.

See https://github.com/riscv/riscv-openocd/pull/711

Change-Id: I8ba27801172ffa955470d2627fa656cad282ee99
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7087
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>