TI Davinci EMAC : Abstract Buffer address translation logic.

When programming the DMA engine, the next pointers must be
programmed with physical address as seen from the DMA master
address space. This address may be different from physical
address of the buffer RAM area. This patch abstracts the
buffer address translation logic.

Signed-off-by: Sriramakrishnan <srk@ti.com>
Acked-by: Chaithrika U S <chaithrika@ti.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>