dmaengine: PL08x: rejig physical channel allocation

Rework the physical channel allocation mechanism to only allocate
physical channels to virtual channels when they're about to be used.
This eliminates all the complexity with holding channels while
descriptors are being prepared, which is completely unnecessary.

This also brings this driver to a state where the generic virtual DMA
code can be used with this driver, and opens up the possibility of
properly scheduling and prioritorising physical DMA channels to
virtual DMA channels.
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>