target-arm: Don't mention PMU in debug feature register

Suppress the ID_AA64DFR0_EL1 PMUVer field, even if the CPU specific
value claims that it exists. QEMU doesn't currently implement it,
and not advertising it prevents the guest from trying to use it
and getting UNDEFs on unimplemented registers.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
---
This is arguably a hack, but otherwise Linux tries to prod
half a dozen PMU sysregs.
