ethoc: use system memory as buffer

This patch enabled the ethoc to allocate system memory as buffer
when there is no dedicated buffer memory.

Some hardware designs may not have dedicated buffer memory such as
on chip or off chip SRAM. In this case, only one memory resource is
supplied in the platform data instead of two. Then a DMA buffer can
be allocated from system memory and used for the transfer.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: David S. Miller <davem@davemloft.net>