target/riscv: Add the Hypervisor extension

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Chih-Min Chao <chihmin.chao@sifive.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

target/riscv/cpu.h

@@ -67,6 +67,7 @@
 #define RVC RV('C')
 #define RVS RV('S')
 #define RVU RV('U')
+#define RVH RV('H')
 
 /* S extension denotes that Supervisor mode exists, however it is possible
    to have a core that support S mode but does not have an MMU and there