arm64: Provide secmon trampoline for restart

arm64: Provide secmon trampoline for restart

If an exception is taken that the secmon won't return
to, there needs to be way to reset that cpu's state
w.r.t. stack usage. Therefore, provide secmon_trampoline
which will reinitialize the exception stack and SP_EL0
and start executing with SP_EL0 like the initial state
of the secmon entry.

BUG=chrome-os-partner:30785
BRANCH=None
TEST=Built and booted to kernel. Also tested when PSCI
     is employed in the kernel.

Change-Id: Ie9f5bbe715dcbcf8b67ea40f9a3a5088ac7aa2ad
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: f1f546ee3e9eca93baaa1ae0437351205bf548a5
Original-Change-Id: Ia3da75e1fa0251c8ea30eb0b0523c8a51c03b917
Original-Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/218922
Original-Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/9096
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)