selftests/powerpc: Add ptrace tests for TM SPR registers

This patch adds ptrace interface test for TM SPR registers. This
also adds ptrace interface based helper functions related to TM
SPR registers access.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>