target/riscv: Add properties for BF16 extensions

Add ext_zfbfmin/zvfbfmin/zvfbfwma properties.
Add require check for BF16 extensions.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20230615063302.102409-2-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
2 files changed