powerpc/mpc85xx:Add support of B4420 SoC

powerpc/mpc85xx:Add support of B4420 SoC

B4420 is a reduced personality of B4860 with fewer core/clusters(both SC3900
and e6500), fewer DDR controllers, fewer serdes lanes, fewer SGMII interfaces and
reduced target frequencies.

Key differences between B4860 and B4420
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B4420 has:
1. Fewer e6500 cores: 1 cluster with 2 e6500 cores
2. Fewer SC3900 cores/clusters: 1 cluster with 2 SC3900 cores per cluster.
3. Single DDRC
4. 2X 4 lane serdes
5. 3 SGMII interfaces
6. no sRIO
7. no 10G

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Signed-off-by: Poonam Aggrwal <poonam.aggrwal@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>