target/arm_adi_v5,arm_dap: introduce pre_connect_init() dap operation

SWD multidrop requires some initialization once before connecting
all daps. Provide an optional pre-connect dap operation.

Change-Id: I778215c512c56423a425dda80ab19a739f22f285
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7542
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/arm_adi_v5.h

@@ -420,6 +420,9 @@ struct adiv5_dap {
  * available until run().
  */
 struct dap_ops {
+	/** Optional; called once on the first enabled dap before connecting */
+	int (*pre_connect_init)(struct adiv5_dap *dap);
+
 	/** connect operation for SWD */
 	int (*connect)(struct adiv5_dap *dap);