powerpc: Use ibm, chip-id property to compute cpu_core_mask if available
Some systems have an ibm,chip-id property in the cpu nodes in the
device tree. On these systems, we now use that to compute the
cpu_core_mask (i.e. the set of core siblings) rather than looking
at cache properties.
Signed-off-by: Paul Mackerras <paulus@samba.org> Tested-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>