MAINTAINERS: Add powerpc drivers to the powerpc section

We'd like folks working on drivers for powerpc to also Cc linuxppc-dev,
so we can be aware of what's going on in drivers and/or review the
changes.

So add patterns to the powerpc MAINTAINERS section to catch some of the
drivers we're interested in.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>