powerpc/smp: Create coregroup domain

Add percpu coregroup maps and masks to create coregroup domain.
If a coregroup doesn't exist, the coregroup domain will be degenerated
in favour of SMT/CACHE domain. Do note this patch is only creating stubs
for cpu_to_coregroup_id. The actual cpu_to_coregroup_id implementation
would be in a subsequent patch.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200810071834.92514-10-srikar@linux.vnet.ibm.com