hw/arm/vexpress, hw/arm/highbank: Don't insist that CPU has reset-cbar property

hw/arm/vexpress, hw/arm/highbank: Don't insist that CPU has reset-cbar property

For the machine models which can have a Cortex-A15 CPU (vexpress-a15 and
midway), silently continue if the CPU object has no reset-cbar property
rather than failing. This allows these boards to be used under KVM with
the "-cpu host" option, since the 'host' CPU object has no reset-cbar
property.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Rob Herring <rob.herring@linaro.org>