powerpc: Add transactional memory paca scratch register to show_regs

Add transactional memory paca scratch register to show_regs.  This is useful
for debugging.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>