selftests/powerpc: Enable range tests on 8xx in ptrace-hwbreak.c selftest

8xx is now able to support any range length so range tests can be
enabled.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/081e3b4e3a17a8ec9fdac46b505e3a29ca15f209.1574790198.git.christophe.leroy@c-s.fr