Deputy maintainer for powerpc

[Paul Mackerras]

I'm going to be on vacation for the next two weeks, and it looks
highly likely that Linus will open the 2.6.27 merge window during that
time, so I'm appointing Ben Herrenschmidt as my deputy maintainer.
During the next two weeks, he'll be the person collecting together any
further powerpc-related commits that need to go upstream, and sending
pull requests to Linus as necessary.

This is the start of a longer-term transition which will ultimately
result in me handing over the powerpc architecture maintainership to
Ben.  Because of various work commitments, this will be a gradual
transition taking probably around 6 months.

Paul.