FVP: Allow building for DynamIQ systems

FVPs that model DynamIQ configuration implements all CPUs in a single
cluster. I.e., such models have a single cluster with more than 4 CPUs.
This differs from existing default build configuration for FVP where up
to 4 CPUs are assumed per cluster.

To allow building for DynamIQ configuration, promote the macro
FVP_MAX_CPUS_PER_CLUSTER as a build option to have it set from the build
command line. The value of the build option defaults to 4.

Change-Id: Idc3853bc95f680869b434b011c2dbd733e40c6ce
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>