KVM: arm64: nv: Unfudge ID_AA64PFR0_EL1 masking

KVM: arm64: nv: Unfudge ID_AA64PFR0_EL1 masking

Marc reports that L1 VMs aren't booting with the NV series applied to
today's kvmarm/next. After bisecting the issue, it appears that
44241f34fac9 ("KVM: arm64: nv: Use accessors for modifying ID
registers") is to blame.

Poking around at the issue a bit further, it'd appear that the value for
ID_AA64PFR0_EL1 is complete garbage, as 'val' still contains the value
we set ID_AA64ISAR1_EL1 to.

Fix the read-modify-write pattern to actually use ID_AA64PFR0_EL1 as the
starting point. Excuse me as I return to my shame cube.

Reported-by: Marc Zyngier <maz@kernel.org>
Fixes: 44241f34fac9 ("KVM: arm64: nv: Use accessors for modifying ID registers")
Acked-by: Marc Zyngier <maz@kernel.org>
Tested-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240621224044.2465901-1-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>