ColdFire: Fix ethernet hang issue for mcf547x_8x

The ethernet hang is caused by receiving buffer in DRAM is not
yet ready due to access cycles require longer time in DRAM.
Relocate DMA buffer descriptors from DRAM to internal SRAM.

Signed-off-by: TsiChung Liew <Tsi-Chung.Liew@freescale.com>