powerpc: Add length setting to set_dawr

Currently we set the length field in the DAWR to 0 which defaults it to one
double word (64bits) which is the same as the DABR.

Change this so that we can set it to longer values as supported by the DAWR.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>