selftests: riscv: Add license to cfi selftest

The cfi selftest was missing a license so add it.

Signed-off-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Link: https://patch.msgid.link/20260309-fix_selftests-v2-4-9d5a553a531e@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>