ARM: tegra: cpuidle: Make abort_flag atomic

Replace memory accessors with atomic API just to make code consistent
with the abort_barrier. The new variant may be even more correct now
since atomic_read() will prevent compiler from generating wrong things
like carrying abort_flag value in a register instead of re-fetching it
from memory.

Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Tested-by: Peter Geis <pgwipeout@gmail.com>
Tested-by: Jasper Korten <jja2000@gmail.com>
Tested-by: David Heidelberg <david@ixit.cz>
Tested-by: Nicolas Chauvet <kwizart@gmail.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>