riscv: Extending cpufeature.c to detect V-extension

Add V-extension into riscv_isa_ext_keys array and detect it with isa
string parsing.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Suggested-by: Vineet Gupta <vineetg@rivosinc.com>
Co-developed-by: Andy Chiu <andy.chiu@sifive.com>
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20230605110724.21391-3-andy.chiu@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>