use master-next meta-fsl-ppc for master-next manifest

Both master and master-next branches exist for meta-fsl-ppc, so use
the meta-fsl-ppc master-next branch in the manifest file on its
master-next branch.

Change-Id: I4e4a8455b4bcea2195159d8c2ae0b5188606c1ed

Signed-off-by: Bob Cochran <yocto@mindchasers.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>