selftests/powerpc: Make reg.h common to all powerpc selftests

Currently there is a reg.h in pmu/ebb that has defines that are useful
in other powerpc selftests so move this up into selftests/powerpc
folder. Also include in utils.h - as this is often used in self tests.
Add in some other useful register defines.

Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>