arm64: KVM: PMU: Inject UNDEF on read access to PMSWINC_EL0

PMSWINC_EL0 is a WO register, so let's UNDEF when reading from it
(in the highly hypothetical case where this doesn't UNDEF at EL1).

Reviewed-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>