This patch adds support to select CCN driver for FVP during build.
A new build option `FVP_INTERCONNECT_DRIVER` is added to allow
selection between the CCI and CCN driver. Currently only the CCN-502
variant is supported on FVP.

The common ARM CCN platform helper file now verifies the cluster
count declared by platform is equal to the number of root node
masters exported by the ARM Standard platform.

Change-Id: I71d7b4785f8925ed499c153b2e9b9925fcefd57a