dmaengine: PL08x: split the pend_list in two

Our behaviour wasn't correct; issue_pending is supposed to be called
before any submitted descriptors are available for processing by the
DMA engine.  Split the pend_list in two, one for submitted descriptors
and another list for issued descriptors.

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>