libpayload arm64: Add library helpers

Add library helpers to access standard arm64 registers. This library also
provides functions to directly read/write register based on current el. So, rest
of the code doesnt need to keep checking the el and call appropriate function
based on that.

BUG=chrome-os-partner:31634
BRANCH=None
TEST=Libpayload and depthcharge compile successfully for ryu

Change-Id: Ibc0ca49f158362d4b7ab2045bf0fbd58ada79360
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 2ca6da580cb51b4c23abdaf04fee2785e5780510
Original-Change-Id: I9b63e04aa26a98bbeb34fdef634776d49454ca8d
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/214575
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/8784
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/arch/arm64/Makefile.inc

@@ -30,6 +30,8 @@
 CFLAGS += -march=armv8-a
 arm64_asm_flags =
 
+subdirs-y += lib/
+
 head.o-y += head.S
 libc-y += main.c sysinfo.c
 libc-y += timer.c coreboot.c util.S