MAINTAINERS: Update powerpc website & add selftests

The selftests under tools/testing/selftests/powerpc are maintained by
us, so add a file pattern for them.

Also drop the www.penguinppc.org link, it's not dead, but the site is
dead (database error). Instead link to the wiki attached to our github,
there is some info there which may be useful, which is better than none.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>