aarch64: Enable early icache and migrate SCTLR from EL3

Initial SCTLR setup done in arm_init_caches for EL3 is now
copied when switching to EL2.

BUG=None
BRANCH=none
TEST=Run coreboot and check for correct SCTLR_EL2 value

Change-Id: I88942ae913cb80c5ca561e5bdd790732dc3348d7
Signed-off-by: Marcelo Povoa <marcelogp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/187468
Reviewed-by: David Hendricks <dhendrix@chromium.org>