dmaengine: mmp-pdma: add number of requestors

The DMA chip has a fixed number of requestor lines used for flow
control. This number is platform dependent. The pxa_dma dma driver will
use this value to activate or not the flow control.

There won't be any impact on mmp_pdma driver.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>