boards: arm64: fvp_base_revc_2xaemv8a: add more cpu nodes

fvp_base_revc_2xaemv8a supports up to 2 clusters and 8 cores, each
cluster has 4 cores.

Due to zephyr supports up to 4 cores, for now, I add another 3 cores,
they all belong to cluster0.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>