Ben is no longer actively maintaining the powermac code, but we know
where to find him if something really needs attention.

The www.penguinppc.org link is dead so remove it.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200224233146.23734-7-mpe@ellerman.id.au