dmaengine: pxa_dma: add support for legacy transition

In order to achieve smooth transition of pxa drivers from old legacy dma
handling to new dmaengine, introduce a function to "hide" dma physical
channels from dmaengine.

This is temporary situation where pxa dma will be handled in 2 places :
 - arch/arm/plat-pxa/dma.c
 - drivers/dma/pxa_dma.c

The resources, ie. dma channels, will be controlled by pxa_dma. The
legacy code will request or release a channel with
pxad_toggle_reserved_channel().

This is not very pretty, but it ensures both legacy and dmaengine
consumers can live in the same kernel until the conversion is done.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>