MAINTAINERS: Update powerpc website & add selftests

[Michael Ellerman]

On Wed, 2016-05-10 at 05:08:16 UTC, Michael Ellerman wrote:
> 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 at ellerman.id.au>

Applied to powerpc next.

https://git.kernel.org/powerpc/c/ad654f25a12d1cbac890d33b7bcc87

cheers