wii: add mem2 dma mapping ops

Some of the devices in the Hollywood chipset of the Nintendo Wii video
game console have restrictions performing DMA transfers to the first
contiguous RAM region (known as MEM1).

For example, up to 3 bytes of the last word of a DMA transfer of a
non-32 bit aligned length to MEM1 may be lost.
Such restrictions do not apply when using the second contiguous RAM
region (known as MEM2).

Add a set of DMA mapping operations which said devices can use to make
sure that DMA transfers are always performed to/from memory buffers
within MEM2.