powerpc: Update powermac MAINTAINERS entry

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