KVM: arm64: Add missing reset handlers for PMU emulation

As we're about to become a bit more harsh when it comes to the lack of
reset callbacks, let's add the missing PMU reset handlers. Note that
these only cover *CLR registers that were always covered by their *SET
counterpart, so there is no semantic change here.

Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>