dma: mmp_pdma: Add the #dma-requests DT property documentation

For pxa based platforms, the number of requestor lines should be
specified, so that the driver can check if the flow control should be
activated (when a requestor line is asked for) or not.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Rob Herring <robh@kernel.org>