powerpc/smp: Move coregroup mask updation to a new function

Move the logic for updating the coregroup mask of a CPU to its own
function. This will help in reworking the updation of coregroup mask in
subsequent patch.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200921095653.9701-11-srikar@linux.vnet.ibm.com