dmaengine: pxa: add a default requestor policy

As what former drcmr -1 value meant, add a this as a default to each
channel, ie. that by default no requestor line is used.

This is specifically used for network drivers smc91x and smc911x, and
needed for their port to slave maps.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Vinod Koul <vkoul@kernel.org>