target/riscv: add common magic

Add common_magic member to struct riscv_info.
Introduce is_riscv() helper.

Change-Id: I1af05988ad869342ba5dc6d4d0ba0ec6a8bf7bc7
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6999
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tim Newsome <tim@sifive.com>

src/target/riscv/riscv.c

@@ -3202,6 +3202,9 @@ struct target_type riscv_target = {
 void riscv_info_init(struct target *target, struct riscv_info *r)
 {
 	memset(r, 0, sizeof(*r));
+
+	r->common_magic = RISCV_COMMON_MAGIC;
+
 	r->dtm_version = 1;
 	r->current_hartid = target->coreid;
 	r->version_specific = NULL;