sh: use declared coherent memory for dreamcast pci ethernet adapter

This patch makes the dreamcast use the recently added declared coherent
memory functions to point out the memory window suitable for dma.

Apart from cleaning up, this gives the dreamcast a proper memory allocator
for pci dma memory.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>