dmaengine: PL08x: move DMA signal muxing into pl08x_dma_chan struct

Move the signal handling out of the physical channel structure into
the virtual channel structure, where it should belong as it has more
to do with the virtual channel than the physical one.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>