atomic.h: change to ATOMIC_DEFINE

Based on review comments for previous patch at
https://gerrit.zephyrproject.org/r/3392

.

The macro changed to ATOMIC_DEFINE since it reserves storage
and doesn't just announce its existence.

Change-Id: Ib6c2b76a219040694926823b94daf6fe779a05d0
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>