FROMGIT: KVM: arm64: Upgrade PMU support to ARMv8.4

Upgrading the PMU code from ARMv8.1 to ARMv8.4 turns out to be
pretty easy. All that is required is support for PMMIR_EL1, which
is read-only, and for which returning 0 is a valid option as long
as we don't advertise STALL_SLOT as an implemented event.

Let's just do that and adjust what we return to the guest.

Signed-off-by: Marc Zyngier <maz@kernel.org>
(cherry picked from commit 46081078
 git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git

 next)
[will: resolved conflict with PMU registers in same way as upstream merge]
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I1c713f3d27cedafd67fc372d25a4ff31581a2ea9
Bug: 178098380
Test: atest VirtualizationHostTestCases on an EL2-enabled device