powerpc/mpc85xx: Enabling CPC conditionally based on hwconfig options

If hwconfig does not contains "en_cpc" then by default all cpcs are enabled
If this config is defined then only those individual cpcs which are defined
in the subargument of "en_cpc" will be enabled e.g en_cpc:cpc1,cpc2; (this
will enable cpc1 and cpc2) or en_cpc:cpc2; (this enables just cpc2)

Signed-off-by: Shaveta Leekha <shaveta@freescale.com>
Signed-off-by: Sandeep Singh <Sandeep@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>