target/riscv: add support for zmmul extension v0.1

Add support for the zmmul extension v0.1. This extension includes all
multiplication operations from the M extension but not the divide ops.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220531030732.3850-1-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.h

@@ -411,6 +411,7 @@ struct RISCVCPUConfig {
     bool ext_zhinxmin;
     bool ext_zve32f;
     bool ext_zve64f;
+    bool ext_zmmul;
 
     uint32_t mvendorid;
     uint64_t marchid;