riscv: hwprobe: Add support for probing V in RISCV_HWPROBE_KEY_IMA_EXT_0

Probing kernel support for Vector extension is available now. This only
add detection for V only. Extenions like Zvfh, Zk are not in this scope.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>