ARM: pxa: add the number of DMA requestor lines

Declare the number of DMA requestor lines per platform :
 - for pxa25x: 40 requestor lines
 - for pxa27x: 75 requestor lines
 - for pxa3xx: 100 requestor lines

This information will be used to activate the DMA flow control or not.

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