arm_dpm: avoid duplicating the register cache

This bug was already attempted to fix in an earlier patch but
merging the "defer-examine" feature caused a regression, which this patch
tries to fix again.

Change-Id: Ie1ad1516f0d7f130d44e003d6c29dcc1a02a82ca
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3951


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

src/target/arm_dpm.c

@@ -972,11 +972,14 @@ int arm_dpm_setup(struct arm_dpm *dpm)
 	arm->read_core_reg = arm_dpm_read_core_reg;
 	arm->write_core_reg = arm_dpm_write_core_reg;
 
+	/* avoid duplicating the register cache */
+	if (arm->core_cache == NULL) {
 		cache = arm_build_reg_cache(target, arm);
 		if (!cache)
 			return ERROR_FAIL;
 
 		*register_get_last_cache_p(&target->reg_cache) = cache;
+	}
 
 	/* coprocessor access setup */
 	arm->mrc = dpm_mrc;