KVM: arm64: PMU: Do not let AArch32 change the counters' top 32 bits

Even when using PMUv3p5 (which implies 64bit counters), there is
no way for AArch32 to write to the top 32 bits of the counters.
The only way to influence these bits (other than by counting
events) is by writing PMCR.P==1.

Make sure we obey the architecture and preserve the top 32 bits
on a counter update.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221113163832.3154370-10-maz@kernel.org