dmaengine: PL08x: convert to use vchan done list

Convert to use the virtual dma channel done list, tasklet, and
descriptor freeing.

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>