powerpc/corenet32: enable DMA in defconfig

By default we enable DMA(CONFIG_FSL_DMA) support
which are needed on P2041RDB, P3041DS, P4080DS,
B4860QDS, etc.

Signed-off-by: Yuan Yao <yao.yuan@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>