powerpc: Update ppc4xx maintainer
powerpc: Update ppc4xx maintainer

Alistair Popple has volunteered to take over maintainership of the ppc4xx
stuff upstream.  Switch the MAINTAINERS entry over to him.

Signed-off-by: Josh Boyer <94755a79aca753e683bb6d0e796d473b797056eb@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <a7089bb6e7e92505d88aaff006cbdd60cc9120b6@kernel.crashing.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs