arm: Make the PMUv3 emulation code arm64 specific

KVM for aarch32 does not exist anymore, PMUv3 is a hardware feature
present only on aarch64 CPUs, the command line option to enable the
feature for a VCPU is aarch64 specific, the PMU code is called only from
an aarch64 function and it compiles to an empty stub when ARCH=arm.
There is no reason to have the PMUv3 emulation code in the common code
area for arm and arm64, so move it to the arm64 directory, where it can
be expanded in the future without fear of breaking aarch32 support.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>