MPC8xxx: Update maintainer entry for Wind River sbc8xxx boards

I've probably got the best chance of getting access to these
boards in order to test things, and since Joe's e-mail is
bouncing, update the MAINTAINERS entry to reflect this.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>