powerpc/cpumask: Refactor /proc/cpuinfo code

This separates the per cpu output from the summary output at the end of the
file, making it easier to convert to the new cpumask API in a subsequent
patch.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>