powerpc: Update contact info in Documentation files

Cody's email address has changed. Update the contact information for
the 24x7 and GPCI counters to the PowerPC developers mailing list.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>