KVM: arm64: Fix MDCR_EL2.HPMN reset value

KVM: arm64: Fix MDCR_EL2.HPMN reset value

The MDCR_EL2 documentation indicates that the HPMN field has
the following behaviour:

"On a Warm reset, this field resets to the expression NUM_PMU_COUNTERS."

However, it appears we reset it to zero, which is not very useful.

Add a reset helper for MDCR_EL2, and handle the case where userspace
changes the target PMU, which may force us to change HPMN again.

Reported-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>