spi/pl022: map the buffers on the DMA engine

The struct device for the DMA engine is the apropriate one to use
when mapping/unmapping buffers. This is because the memory which
is addressable by DMA is determined by the DMA engine rather than
the device.

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>