boot: zephyr: Add check of RESETREAS for nRF SoC

Only execute serial recovery if valid pin condition is detected
during power up reset or pin reset on nRF devices with RESETREAS
register. This is to prevent going into serial recovery when
waking up from power off mode with the same pin conditions that
is configured to trigger serial recovery.

Signed-off-by: Jon Helge Nistad <jon.helge.nistad@nordicsemi.no>