target: cortex_m: fix display of DWT registers

Commit 16b4b8cf54 ("Cortex-M3: expose most DWT registers") added
the DWT registers to the list of CPU registers.
The commit message from 2009 reports the reason behind this odd
mixing of CPU and DWT registers.
This feature got broken in 2017 with the introduction of the field
struct reg::exist and its further use in the code. As result, the
command 'reg' on a target Cortex-M reports only the core registers
and then the header line
	===== Cortex-M DWT registers
not anymore followed by the DWT registers.

Fix it by tagging each DWT registers as existing.

Change-Id: Iab026e7da8d6b8ba052514c3fd3b5cdfe301f330
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: b5964191f0 ("register: support non-existent registers")
Reviewed-on: https://review.openocd.org/c/openocd/+/8198
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins

src/target/cortex_m.c

@@ -2351,6 +2351,7 @@ static void cortex_m_dwt_addreg(struct target *t, struct reg *r, const struct dw
 	r->value = state->value;
 	r->arch_info = state;
 	r->type = &dwt_reg_type;
+	r->exist = true;
 }
 
 static void cortex_m_dwt_setup(struct cortex_m_common *cm, struct target *target)