powerpc/512x: add MPC8308 dma support

MPC8308 has pretty much the same DMA controller as MPC5121 and
this patch adds support for MPC8308 to the mpc512x_dma driver.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Acked-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>