New maintainer for powerpc architecture

After looking after the powerpc architecture part of the Linux kernel
for 7.5 years, I am now stepping down, and Ben Herrenschmidt will be
taking over as powerpc maintainer.

I'm not abandoning Linux or PowerPC though; I'll still be working on
various powerpc/linux things, including being the platform maintainer
for pSeries, working on performance monitor infrastructure (whichever
one gets in, in the end), etc.

Paul.