arm64: Correctly unmask asynchronous SError interrupts

Arm CPUs have always had an odd feature that allows you to mask not only
true interrupts, but also "external aborts" (memory bus errors from
outside the CPU). CPUs usually have all of these masked after reset,
which we quickly learned was a bad idea back when bringing up the first
arm32 systems in coreboot. Masking external aborts means that if any of
your firmware code does an illegal memory access, you will only see it
once the kernel comes up and unmasks the abort (not when it happens).

Therefore, we always unmask everything in early bootblock assembly code.
When arm64 came around, it had very similar masking bits and we did the
same there, thinking the issue resolved. Unfortunately Arm, in their
ceaseless struggle for more complexity, decided that having a single bit
to control this masking behavior is no longer enough: on AArch64, in
addition to the PSTATE.DAIF bits that are analogous to arm32's CPSR,
there are additional bits in SCR_EL3 that can override the PSTATE
setting for some but not all cases (makes perfect sense, I know...).
When aborts are unmasked in PSTATE, but SCR.EA is not set, then
synchronous external aborts will cause an exception while asynchronous
external aborts will not. It turns out we never intialize SCR in
coreboot and on RK3399 it comes up with all zeroes (even the reserved-1
bits, which is super weird). If you get an asynchronous external abort
in coreboot it will silently hide in the CPU until BL31 enables SCR.EA
before it has its own console handlers registered and silently hangs.

This patch resolves the issue by also initializing SCR to a known good
state early in the bootblock. It also cleans up some bit defintions and
slightly reworks the DAIF unmasking... it doesn't actually make that
much sense to unmask anything before our console and exception handlers
are up. The new code will mask everything until the exception handler is
installed and then unmask it, so that if there was a super early
external abort we could still see it. (Of course there are still dozens
of other processor exceptions that could happen which we have no way to
mask.)

Change-Id: I5266481a7aaf0b72aca8988accb671d92739af6f
Signed-off-by: Julius Werner <jwerner@chromium.org>
---
M src/arch/arm64/armv8/cpu.S
M src/arch/arm64/include/armv8/arch/cache.h
M src/arch/arm64/include/armv8/arch/lib_helpers.h
M src/arch/arm64/transition.c
M src/arch/arm64/transition_asm.S
5 files changed, 59 insertions(+), 129 deletions(-)