sh: dma: Make PVR2 DMA configurable.

With arch/sh/drivers/dma/ always being built, the Dreamcast DMA engines
are being unconditionally built in, regardless of whether the DMA API is
enabled or not. This is a regression from previous behaviour, but there
is not much advantage in building them all in unconditionally regardless.
Add a new config option to make it optional, and update the only user of
it to reflect that.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>