MIPS: Add DMA declare coherent memory support

The ohci-sm501 driver requires dma_declare_coherent_memory().  It is used
by the driver's local memory allocation with dma_alloc_coherent().

Tested on TANBAC TB0287(VR4131 + SM501).

[Ralf: Fixed reject in dma-default.c and removed the entire #if 0'ed block
 in dma-mapping.h instead of just the #if 0.]

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>