MAINTAINERS: Update Linux for Freescale PowerPC

About a year ago I began taking patches, technically as Kumar's
assistant -- but since then all of the pull requests for this area have
come from me, and I've been doing most of the reviews.  Update
MAINTAINERS to reflect this.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>