arm64: Add conditional read/write from/to EL3 assembly macros.

arm64: Add conditional read/write from/to EL3 assembly macros.

Some registers are available only at EL3. Add conditional read/write functions
that perform operations only if currently we are in EL3.

BUG=chrome-os-partner:33962
BRANCH=None
TEST=Compiles and boots to kernel prompt.

Change-Id: Ic95838d10e18f58867b6b77aee937bdacae50597
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 62a0e324a00248dba92cb3e2ac2f4072d0e4e2a7
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Change-Id: Ia170d94adb9ecc141ff86e4a3041ddbf9045bc89
Original-Reviewed-on: https://chromium-review.googlesource.com/231549
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/9538
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>