atomic: Add helper to declare atomic_t arrays for bit fields

The atomic_t API supports arbitrary sized bit fields which are not
limited to a single atomic_t variable. This patch introduces a macro
to help declare right-sized atomic_t arrays given the number of bits
needed.

Change-Id: I226f6312b642551fc492df29d24764222f45ac83
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>