target: cortex_m: fix polling for target kept under reset

target: cortex_m: fix polling for target kept under reset

In multi-target SoC not all the targets are running simultaneously
and some target could be powered off or kept under reset.
Commit 4892e32294c6 ("target/cortex_m: allow poll quickly get out
of TARGET_RESET state") does not considers the case of a target
that is kept in reset and expects the target to change state from
TARGET_RESET immediately.
This causes OpenOCD to log continuously:
	Info : [stm32mp15x.cm4] external reset detected
	Info : [stm32mp15x.cm4] external reset detected
	Info : [stm32mp15x.cm4] external reset detected

Read again dhcsr to detect the 'stable' reset status and quit,
waiting for next poll to re-check the target's status.

Change-Id: Ic66029b988404a1599bb99bc66d4a8845b8b02c6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 4892e32294c6 ("target/cortex_m: allow poll quickly get out of TARGET_RESET state")
Reviewed-on: https://review.openocd.org/c/openocd/+/8399
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins