powerpc/B4: Merge B4420 and B4860 in config_mpc85xx.h

B4420 is a subset of B4860. Merge them in config_mpc85xx.h to simplify
the defines.
- Removed #define CONFIG_SYS_FSL_NUM_CLUSTERS as this is used nowhere.
- defined CONFIG_SYS_NUM_FM1_10GEC to 0 for B4420 as it does not have 10G.

Also move CONFIG_E6500 out of B4860QDSds.h into config_mpc85xx.h.

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