fix XScale register access

since b5964191f0, all XScale-specific registers were missing, breaking
config scripts.

Change-Id: Ia56f3ca17500ba54bd08f417e9a5aaaa8a1be8c4
Signed-off-by: Michael Schwingen <spam-openocd@discworld.dascon.de>
Reviewed-on: http://openocd.zylin.com/5136
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/target/xscale.c

@@ -2895,6 +2895,7 @@ static void xscale_build_reg_cache(struct target *target)
 		(*cache_p)->reg_list[i].size = 32;
 		(*cache_p)->reg_list[i].arch_info = &arch_info[i];
 		(*cache_p)->reg_list[i].type = &xscale_reg_type;
+		(*cache_p)->reg_list[i].exist = true;
 		arch_info[i] = xscale_reg_arch_info[i];
 		arch_info[i].target = target;
 	}