powerpc: Pull out cpu_core_mask updates into a separate function

This factors out the details of updating cpu_core_mask into a separate
function, to make it easier to change how the mask is calculated later.
This makes no functional change.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>