FVP: change the method for translating MPIDR values to a linear indices

FVP: change the method for translating MPIDR values to a linear indices

x3 will be assigned by the folloing instructions.
So the first instruction is not needed any more.

old method:
  (ClusterId * FVP_MAX_CPUS_PER_CLUSTER)
+ (CPUId * FVP_MAX_PE_PER_CPU)
+ ThreadId

it should be
  (ClusterId * FVP_MAX_CPUS_PER_CLUSTER) * FVP_MAX_PE_PER_CPU
+ (CPUId * FVP_MAX_PE_PER_CPU)
+ ThreadId

which can be simplified as:
(ClusterId * FVP_MAX_CPUS_PER_CLUSTER + CPUId) * FVP_MAX_PE_PER_CPU + ThreadId

Signed-off-by: Wang Feng <feng_feng.wang@spreadtrum.com>