tcl: stm32mp15x: fix "reset halt" on CM4 in engineering boot

tcl: stm32mp15x: fix "reset halt" on CM4 in engineering boot

The state machine of cortex-m have to pass through a set of state
before it get in "halted".

Add one more "arp_poll" to achieve the proper state during a
"reset halt" command in engineering boot.

Change-Id: I90828bf20ef75bd4018f8b911f727ae69c4d6e8f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5649
Tested-by: jenkins
Reviewed-by: Richard Braun <rbraun@sceen.net>