arm64 libhelpers: Add helper functions for writing sp_elx

arm64 libhelpers: Add helper functions for writing sp_elx

BUG=chrome-os-partner:30785
BRANCH=None
TEST=Compiles successfully

Change-Id: Id9367b1fc836b7b8c8fd15b372673853493f67d4
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 561e9c4ec2b0657846d50bbc893ef7541230f932
Original-Change-Id: Ie950e893b01456c23af14304bd4dd8f61af9f244
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/216905
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Tested-by: Furquan Shaikh <furquan@chromium.org>
Original-Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/9064
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>