selftest/powerpc/ptrace: Cleanup duplicate macro definitions

Both core-pkey.c and ptrace-pkey.c tests have
similar macro definitions, move them to "pkeys.h"
and remove the macro definitions from the C file.

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241216160257.87252-3-maddy@linux.ibm.com