libpayload: arm64: Conform to new coreboot lib_helpers.h and assume EL2

This patch adds the new, faster architectural register accessors to
libpayload that were already added to coreboot in CB:27881. It also
hardcodes the assumption that coreboot payloads run at EL2, which has
already been hardcoded in coreboot with CB:27880 (see rationale there).
This means we can drop all the read_current/write_current stuff which
added a lot of unnecessary helpers to check the current exception level.

This patch breaks payloads that used read_current/write_current
accessors, but it seems unlikely that many payloads deal with this stuff
anyway, and it should be a trivial fix (just replace them with the
respective _el2 versions).

Also add accessors for a couple of more registers that are required to
enable debug mode while I'm here.

Change-Id: Ic9dfa48411f3805747613f03611f8a134a51cc46
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/29017
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Patrick Rudolph <patrick.rudolph@9elements.com>

payloads/libpayload/arch/arm64/Makefile.inc

@@ -30,8 +30,6 @@
 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