ish: refactor IPC usage of REG macros into registers.h

This is the final CL needed to resolve b:130573158.

BUG=b:130573158
BRANCH=none
TEST=arcada_ish functions as normal after changes

Change-Id: Ia4cc9bfa95938b9f57fc1cd241cd6821b42a3ce6
Signed-off-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1578435
Reviewed-by: Jett Rink <jettrink@chromium.org>
4 files changed