powerpc/512x: add ifm ac14xx board

Add dts file for ac14xx board and its board compatible
string to the generic mpc512x board match list.
Also add phandle to the dma DT node since there is a change
(for MPC5121 SDHC DMA support) merged via linux-mmc tree
with reference to the dma controller node in the sdhc node.

Signed-off-by: Anatolij Gustschin <agust@denx.de>