armv7: Move Exynos from 'cpu' to 'soc'.

The Exynos family and most ARM products are SoC, not just CPU.

We used to put ARM code in src/cpu to avoid polluting the code base for what was
essentially an experiment at the time. Now that it's past the experimental phase
and we're going to see more SoCs (including intel/baytrail) in coreboot.

Change-Id: I5ea1f822664244edf5f77087bc8018d7c535f81c
Reviewed-on: https://chromium-review.googlesource.com/170891
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Ronald Minnich <rminnich@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>
(cherry picked from commit c8bb8fe0b20be37465f93c738d80e7e43033670a)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6739
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>