riscv: kconfig: Normalize architecture name spelling

It's RISC-V that is the official name, not RISCV.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Rick Chen <rick@andestech.com>